I had the same problem and have two fixes, one is when putting the braces on take your time and work out a routine that works, mine is to put one brace on and tension it, then put the other on and tension it , do a couple of squats to settle them (this also expands your muscles and pulls the strap into place) then go back and retension as tight as is comfortable. Sounds more work but it isn't really. The second bit of advice is to make sure your pants are well pulled up around the knee, especially if the braces make them a tight fit, it took me a while to realise that when I bent my knees my pants were pulling the brace down.