I can relate to the problems you are having with your Mum and her inability to communicate her needs/pain. I have the same problem with my sister because she is in the advanced stages of this awful disease.
The way we do it is --- If you think she wants/or is trying to tell you something. Ask a question, for instance, "are you in pain" if you are squeeze my hand. I ask questions for the various scenarios drink, toilet, need to lay down etc. It is a process of elimination until you get to the right one but when someone is in this stage you have no other option. We have tried a sound board and flash cards but my sister has gone beyond being able to use them. There is no easier answer you have to find any way you can to at least retain some way of communicating . Like everything with PSP it is trial and error and thinking outside "the box".
Good Luck and keep trying.