Yes, since PPTP only looks at the (public) IP of endpoints you can't have multiple endpoints behind the same NAT IP.
Do we have multiple people with the same user name (and is it set to allow multiple connections for 1 user)?
Is your PPTP Pool big enough?
Are your Timeouts (if applicable) too short?
What I am trying to say is there is no obvious reason for this problem, we need to take a closer look at the specifics of your configuration and testing.