Advanced Poll IP logging bugs?

When I enable IP logging and set the houres to maximum
(so they can only vote once per monthly poll)
some people write me that they cannot vote at all,
even when they haven't voted yet....

When I disable IP logging, they can vote... but unlimitted..
(thats normal, because thats why you turn off IP loging)

But how can I fix the problem that some people can't vote
at all..

and what causes this problem anyway?