Hi Tasrine,
i havent found this problem, but i think its possible when you have it set to infinite ban, people on dsl etc where their IP's change makes the infinite ban a waste of time and space in your banlist,
the best way is to keep the IP's on there of banned people that dont change IP's, remove them that do.
another tip is to check on hostnames as an on-entry as banning a changed IP only stops someone else getting in when the unwanted name has changed IP.