My forum does this too sometimes. I never figured out why Now I send a email to the new member as soon as they join so they can contact me if they have trouble. Usually they are fine.
I have found my forum does not like usernames with special characters in them. Someone with a name like *NEW//---p1ay3r* won't be able to register at my forum usually. Fine by me too. LOL. I hate it when people jack up their names so bad I can hardly tell what it says. LOL.