C2422HE, won't shut off, two PC setup

I have a Dell desktop and Dell laptop hooked up to one monitor (C2422HE). This is a new setup. My previous setup (before I had this monitor) was all hooked up through a KVM switch. Now that I have this new monitor, I have everything plugged into the monitor. I have 'shortcut button 1' set to my laptop (C to C) and 'shortcut button 2' set to the PC with the DP cord. 

Typically I "work" (for my job) on my laptop and sign out of Windows when I'm finished. I flip over to my PC and check personal email, etc. and that's where I would hit the power button on the monitor. (I don't power down either computer when done.) In my old setup, the old monitor would turn off immediately. With this new setup the monitor won't immediately shut off when I press the power button, and often it won't shut off at all.  

Sometimes there's a delay of 1-3 minutes. Once I pressed it and thought it would turn off, so I left the room; woke up several hours later to a screen with a bunch of horizontal lines over 95% of the screen (the top area was still clear.) Sometimes I press & hold the button; sometimes I press then press again. Sometimes I think nothing's happening so I start to google to find a solution, and then it suddenly turns off. 

One thing I didn't think would matter, but maybe it does - after hooking up all my new stuff and confirming everything worked as planned, I put my work station back together and ended up using the power cord from the prior monitor because I had all my wiring situated how I wanted it. 

Any thoughts on this would be appreciated. Just an FYI, I'm a novice at computers so if I haven't included all necessary info, just let me know. Thanks!!