New secondary monitor has lower resolution and refresh rate, why?

Dear all

I have got exactly the same problem with 2721DE monitors

Rgds

@plfernan  Can you provide more detail about the exact equipment you're using and how it's all connected?  The post from @JitheshS  provided a lot of that, but crucially omitted the laptop model that was being used.  But if you've got your U2721DE displays set up in a daisy chain with the first display connected to your system via USB-C to USB-C cable, and your system only supports DisplayPort 1.2/HBR2 over USB-C rather than 1.4/HBR3, then the fix to this issue is to go into the on-screen configuration menu of the first display and change its USB-C Prioritization setting to High Resolution rather than High Data Speed.  That will cause you to only have USB 2.0 data speeds to the display, so you may want to avoid connecting USB 3.x peripherals to the USB ports on the display.  If you want dual 1440p and USB 3.x, you'll either need to run a separate USB 3.x cable for data or connect each display directly to your system.  With systems that only support DP 1.2/HBR2 over USB-C, you don't have enough bandwidth to run dual 1440p and USB 3.x all at the same time over a single cable, at least not without using Thunderbolt 3, but the U2721DE isn't a Thunderbolt display.  I wrote an explainer post here if you're actually curious about how this all works.

As for @JitheshS , once again you didn't specify your laptop model, which is a key bit of information.  But based on what you did describe, it sounds like there's one of two possible causes here:

• You're using a laptop that supports Thunderbolt 3 but that isn't operating in actual Thunderbolt mode with the dock.  If the dock is operating in USB-C backward compatibility mode, then since your system only supports DP 1.2/HBR2 based on the Intel GPU model you mentioned, you won't be able to run dual 1440p and USB 3.x over regular USB-C simultaneously.  The fix in that case would be to make sure the dock is running in actual Thunderbolt mode, which will typically require approving it in Thunderbolt Control Center.
• Otherwise, if your laptop doesn't support Thunderbolt 3, then the dock will only be capable of operating in USB-C backward compatibility mode, and since your system only supports DisplayPort 1.2/HBR2 rather than DP 1.4/HBR3, this limitation is expected.  A regular USB-C system running DP 1.2/HBR2 can only run dual displays up to 1920x1200 each or a single QHD/1440p display when paired with a regular USB-C dock that is also configuring the link to support USB 3.x data, as basically any modern dock would.  If you want dual QHD, you either need a system and dock that support DP 1.4/HBR3 over USB-C, or you need a system and dock that both support Thunderbolt.  (Or technically you could run dual QHD from a DP 1.2/HBR2 system through a regular USB-C dock if the dock only ran USB 2.0 data, but even if that would be acceptable, those docks are pretty rare.)  This all has to do with how USB-C and Thunderbolt work and how much bandwidth is supported by these DisplayPort standards.

@jphughan I am having the same issue with my second monitor. I am running:

• 2 Dell S2721D monitors (capable of QHD)
• a Hiearcool USB C dock: https://www.amazon.com/gp/product/B09QSQ7BSJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1 
• both monitors are hooked up to the dock via HDMI cables
• Laptop is Dell Inspiron 7415 2-in-1 

Any suggestions?

Thanks!

@hazlefinn  Short version: If you want to make the most of your displays, you should connect only one of them to that dock and connect the other directly to the HDMI 1.4b output on your system. That will allow both to run at QHD 60 Hz simultaneously. Obviously that's another cable to deal with, but that's still arguably much better than either running your QHD displays at 30 Hz or running them at 1080p, which will be your options if you continue trying to run both displays through that dock.

Longer version:
That system's Setup and Specifications document indicates that it uses an AMD Ryzen CPU. I'm not familiar with which models of AMD's CPUs support which DisplayPort revisions over USB-C, and the "External Ports" section of that document here just mentions that the USB-C port supports "DisplayPort" -- sort of annoying given that the very same section indicated the exact HDMI revision supported on the HDMI port.  (Fyi, USB-C will always output video as a DisplayPort signal, even if you use a dock that later converts it to HDMI or VGA.)  But if the system only supports DP 1.2 over USB-C, then it would only support single QHD 60 Hz (or 4K 30 Hz) or dual 1080p 60 Hz. If it's a DP 1.3-based system or better, then it would be able to support single 4K 60 Hz or dual QHD 60 Hz (or 4K 30 Hz)....when paired with suitable peripherals.

Which brings me to that dock you're using.  Its product description includes this: "2 x 4K-HDMI ports support resolutions up to 3840 x 2160@30Hz with single HDMI displays attached".  It's a bit odd that it says "single" and then "displays" (plural), but based on my knowledge of the underlying technologies and their capabilities, and the fact that the specs don't mention 4K 60 Hz support in any context at all, the meaning of that item would be that each individual HDMI output on that dock supports up to 4K 30 Hz, but you can only do that when only one HDMI output is in use.  And the lack of any mention of 4K 60 Hz tells me that the dock only supports receiving DP 1.2 over USB-C, not 1.3 or better.  And that would mean that you can't run dual QHD 60 Hz displays through that dock.  It simply won't support that level of video bandwidth from the system even if the system would support providing it, but that part isn't clear to me either.

Unfortunately, my uncertainty on the system's capabilities means I can't tell you for sure if upgrading to a dock that supports newer video standards would remove this limitation for you. It might, but it's also possible that you'd still be limited to your current setup due to a system-side limitations of only supporting DP 1.2 bandwidth levels over USB-C.  And that's why I suggested simply connecting one of your displays to the system's built-in HDMI output as a surefire way to achieve full functionality without spending any more money. Hopefully this helps!

@jphughan you are the man! thank you for the quick and detailed response. The direct connection of one of the monitors directly to the laptop HDMI output solved the issue. I am in your debt good sir, cheers! 

@hazlefinn  Glad I could help, and enjoy your displays!