I'm using a MacBook Air from late 2010 or so, and I've recently noticed that my copy/paste abilities have become significantly reduced. Within an app itself it usually works without a hitch, but occasionally if I try to paste between apps (like, say, use a messaging app to send a link to someone), it'll usually either paste the last thing I copied within the app or nothing altogether. Right now, I'm just looking for some kind of quick, lasting fix, as this is making my usual workflow significantly more difficult.
Occasionally, it is possible for keyboard service to either die or become unresponsive. I've had this too, although I'm not really entirely sure under what circumstances. The following restarts it "gently", and may help you:
launchctl stop com.apple.pboard launchctl start com.apple.pboard