I could not find out how to contact the plugin author directly so hopefully they monitor the tags on posts here. I did a search here and online and found a few others asking the same question. Those did not have answers or suggestions so I'll try to explain the issue.
Over that past couple of months I have been made aware that the tremendous CFC plugin is catching some legitimate comments. I have been using CFC for more than a year and have not had this complaint until recently. Some of the blocked comments are from regular commentators. One has commented successfully 120 times previous to his comments being blocked. I was able to contact and ask several of the people unable to comment and they all have cookies enabled so that is not the problem.
I have disabled the plugin for now, but don't want to keep it disabled as my spam count immediately went up by at least a factor of 50!