Editing clips isn't working correctly

I'm new to this but program doesn't seem to be working correctly. When I try to shorten a clip on either side it does not show in the monitor above, so very difficult to try and edit at the right spot that I'm trying for. Also when trying to clip using the scissors icon the clip doesn't clip. It was working fine when I was doing guided tutorials yesterday but when I started a project today it stoped working. I have tried in expert and in guided. Have a pressed some wrong button or what can I do to fix this?