Well, I can offer a few suggestions:
Make sure you do not have any open web browser windows/tabs at all to the account that isn't working. You may even want to reboot your system just to make sure there are no residual connections (particularly if you ever went into Gmail on that account since your last reboot).
If that's not the problem, then carefully compare settings on the Google Voice configuration pages between the account that does work and the account that doesn't. You might have missed something, though I am not sure what it could be.
There's really not that much that can go wrong in the OBi device setup, assuming you didn't try any customizations to the configuration, and used either the wizard or the OBiTALK portal to initially configure the account. If you entered correct account information, that should work, assuming the Google Voice account is correctly configured and is working.
If all else fails, make sure the Google Voice account isn't messed up. Try changing the forwarding to another phone temporarily and make sure you can receive calls when you dial the GV number. If you can, go into the Gmail account portal (something you normally should not do, but at this point you're at the "grasping at straws" stage) and see if you can place a call from within Gmail that will ring the other phone, then connect through. Once you know that works, change the forwarding back to Google Chat (and be sure to log out of that account!).
And always check for typos when entering account information, particularly passwords (which are usually blanked out so you can't readily see any mistakes).
That's about all I can think of.