Hi @rodders1947, @floraoliviaa,
It seems that you’re experiencing a common issue related to missing windows/applications in windows 10 and 11, this event usually happens when your app/window is accidentally placed inside or under the taskbar while using dual display through physical (HDMI/DVI/VGA) or network connection in extend mode which means spacedesk server/viewer is not the cause of the issue. I have tried reproducing a missing window in this video. https://drive.google.com/file/d/1–zutXirZx3soOS1h5Ouf-hD1TU_BVsC/view?usp=sharing
the video showcase that some applications that goes missing in multiple display are usually under the taskbar and you won’t be able to access it again or move it unless you restart the application.
you can try this workaround to find/make those missing applications visible in your screen to access it:
if you are using windows 11 in your primary screen:
-press Windows Key + Tab -> (all of your running application will appear in both screen) hover your mouse to your missing window/map and press right click -> press snap right or snap left.
if you are using windows 10 in your primary screen:
-hover your mouse in the taskbar and press right click -> choose “Cascade Windows”. (all of your active applications will appear in your screens including the missing one)