Have you lubed up the hammer mechanism? Short of taking it apart and putting it together (fiddly to say the least for most gbbs) it might be the only thing to do
Edit, just read the post above properly. Maybe an internal lube could help.
Otherwise, how does the slide feel moving back, is it stiffer than usual (indicating resistance in a moving part, or a jammed part) is there any dirt or crap where there shouldn't be?