The best thing to do is to check the logs. There has to be something there that gives a hint about the problem.
Other than that, a few things comes to my mind:
1) You have verified the settings for the accounts, but are you sure they have been created in exactly the same way? Maybe you have missed something, how about creating new accounts, carefully noting every step and see what works & what doesn't.
2) Passwords: A few times I have had problems with some characters in keys/passwords. Try changing password for a "not-able-to-connect" user and use only alphabetical.
3) "...can login as one user but not the other..." You did logout the one user first, right? What I mean is, you can't easily (if at all?) have multiple pptp connections open at the same time.
4) You try from same computer in same place?
The reason for this question is that pptp is a somewhat troublesome protocol, with many firewalls (Cisco ASA 5505 for example) you have to specifically configure it to allow GRE pass-through.
And last but not least: you should consider moving away from pptp, it is not a good vpn solution. The protocol is compromised, meaning it is not considered secure.
There are several other vpn solutions out there, personally I use OpenVPN whenever I can choose.