If you are using the AntiSpam Bee plugin on your WordPress blog, then you should leave the option “Look in the local spam database” off. More often these databases end up adding genuine IP addresses which are usually dynamic and thus easily fall prey to spammers. The ISPs are usually very slow in getting these IPs cleaned up from public spam lists. This has been the case with several ISPs in India. The ISP which I use, MTNL has huge set of IPs listed in the public spam lists. They don’t bother to get them removed. And as a result the IP addresses I get are a part of almost every other spam list which prevents me from commenting or accessing so many sites.
Same case can happen with the users on your blog. Just today I spotted a genuine comment in the Spam list with the reason mentioned by AntiSpam Bee as “Server IP”. As suspected the IP address was found in some of the spam lists but the comment in no way was spam. If your blog also receives a substantial number of false negatives, you should check whether this option is on or not and switch it off if it is.