I have had this problem a few times, and couldn't figure out what was happening..I 'thought' I had copied, but nothing happened. I finally decided it was simply that I hadn't managed to do the copy...perhaps because I had hit some key and lost 'focus' (was not in the window I thought I was) or because hadn't actually HIT the copy command correctly...

I usually try some other copy/paste trials in some other place until it seems ok again....sometimes you just never know..