WD15 Dock Switch Screen Intermittent

The WD15 connected with Dell 5490 via USB-C smoothly (PC screen to Second screen only instantly). But when changing to Dell 7440 (refreshment program), it was intermittently switched to second screen once USB-C is plugged in. The temporarily solution is to open lid and do the log in first then manually hitting the switch screen button on the key board (for 5490 screen can be switch without open the lid). I need your support on this issue. It is not prevent using the dock but it took lots of time.