It would depend how you have things set up. what integration and what usergroup settings.
Example if you are integrating with VB3 and a user registers and does not confirm there account then they are not verified right so even though they can login they cant post in a normal vb
unless you allow unregistered posting so same thing goes for Photopost they would have no upload link unless you allow it.
So for these few users you need to find out what exactly the group they are in to determine your issue although I am pretty sure the above explains your issue to a tee