I have the same router mounted in a table. Using only the big knob to raise it is difficult, because it's lifting on just one side, which causes binding. I generally turn the knob with one hand (squeezing the ring in with my fingers) and at the same time apply a little upward pressure with my other hand. If you are using the same technique (with height adjustment unlocked, of course) I don't know what the problem could be. Does the router plunge smoothly when not in the table?
Incidentally, I've installed a Wixey digital guage, and find I can get the desired height using the big knob only.