Unlocked Pulse not getting phone network

Couple of months ago I gave my T-Mobile Pulse to my wife to use as I had bought myself a new phone. As she had a contract with Talk Mobile I purchased a code to unlock the phone so that she could use her sim in the Pulse. Everything worked fine for 2 months but today it's no longer connecting to the phone network.
I put my T-Mobile sim back in the Pulse and it worked fine and I put her Talk Mobile sim back in her old phone and it worked fine. For some reason her sim will no longer work in the Pulse.
It's not asking for the code to be inputted again like it asked when the sim was first inserted. Is it possible that an unlocked phone can suddenly lock itself again, or has anyone got any other ideas what may be happening?
Thanks for your help in advance.