Sketchup 2015 second display

when using a second display from time to time what is displayed on the
second display jumps to the firts display. In sketchup 14 I never had this problem.

I operate with SU on my primary monitor and keep the windows I use most on my secondary monitor. Ever since SU 7 the arrangement has stayed that way; however the last two weeks both the SU drawing window and the menu windows appear on my primary monitor - I can of course move them to my second monitor, but when the session ends and I reopen SU it goes back to the primary monitor. How do I lock them? I’ve tried the above solutions but no joy.

Operating system is Windows 7,using Su15 latest build. I posted this on SketchUcation, no replies.