They didn't work for me at first, either. I realised that they began working when I pressed the buttons faster than before. Perhaps you could try pressing them faster, yet still making sure one is pressed directly after another and that you don't accidentely press more than one at a time. Also make sure the shoulder buttons are completely held down. In any case, just keep trying and trying and they should work.
You don't need to be in any special area or be paused or anything, but what I found helps is if you don't have any Psi Powers active that do something regardless of whether you focus on someone or not (so try having Telekinesis, Pyrokinesis, and Clairvoyance as your powers).