Promise of increased pay never materialised

My partner was asked to change her role to different department in the same workplace. It was explained that after few weeks of training they would increase her pay as it is totally different role. They have not changed her contract. My partner has spoken to the manager and sent a few emails, but they keep making excuses that they will look into it and they leave it as that.  Is this wrong or is it her fault for accepting the role without asking for a new contract and pay rise?
This has been over three years now and she is still working under the old contract and job title.