Scroll randomly doesn't work in 2 guest win 7 when switching between them.

I have parallels desktop 8.
When i switch between my 2 victuals win 7, sometime scroll doesn't work.
It's repaired after clicking on scrollbar.
How can i fix it without clicking on scrollbar?

 

Hello Jesprit, please shut down Windows virtual machine, open its configurations and go to Hardware > Boot Order > and paste in Boot Flags box: devices.usb.mouse=0 and see if this helps. Thanks!