I can see visitors as registering.
Stop forum spam, timer and Q/A are working hard, they cannot get through.
But why couldn't they get through, what stopped them?
Q/A ? SFS ? Timer?
Is it possible to get the reason(s) of the rejection?
Q/A and timer wouldn't reject a user without giving them a chance to resolve the issue.
SFS may outright reject them if they meet enough flags. Otherwise, if the user is moderated (and you don't approve everyone), it's always a spam-related trigger.