I, and a few other players I know, have noticed that we're sometimes unable to block or use a Sign, even tho Geralt is standing still (i.e., not in the middle of another animation) and has plenty of Vigor. This has only been a temporary problem for me; I can generally block/Sign again once I roll or run way from the action and turn back to it. But something's clearly going on, and it's definitely tied to combat - because I can block on/off over and over and over with no problems whenever I'm not in combat. Maybe something to do with the targeting system? I have no idea, but it's very frustrating.
This and the lack of FOV configurability are my only significant issues with this great game, but the combat unresponsiveness in particular is a big one.
I'm not clear on whether this issue is related to what some other posters in this thread have identified. Any ideas?