What am I doing wrong with my pressure cooker?
I use a pressure cooker quite often and its all about the cut of the meat.  Much like slow cooking, using a often cheaper, fatty cut with lots of connective tissue - a chuck roast for example, yields much, much better results.  Lean cuts of meat are make for fast searing and that's about it.  If you try to coax them to be fall apart fork tender, you'll fail - those are the cheap, fatty cuts that go that way.