I've tried three different facebook like button plugins, and on all three, they only work some of the time. I have no problem with "tweet" buttons or any other social network button. Add to/any works, but not their fb like button, either.
Either you click it and it appears to +1, but doesn't stick, or it outright says "error" in red.
The inconsistency of it working some of the time is the most puzzling part, because I don't seem to be changing any variables.. it happens on its own.