I have had a few occasions in the past where the eye dropper wouldn't select certain areas of my image (tho I haven't had the problem in a long while). And I came to the conclusion that the file itself had been corrupted some how. So I saved it to .psd instead of .riff and reopened it and that seemed to sort it out. Saving it back to a .riff wasn't a problem.

However I couldn't say that this is what your problem is. I think this occurred using version IX. Can't remember it happening in X.