Try a little oil for sticky parts. I use something from radio hack that has teflon bits in it. It comes in a small pen style applicator with a needle type tip. Works very nice.

Also check to make sure your blade gripsare nice and free to move, disconnect linkages and try twisting them. If theyre too tight you may need to remove a shim.

Also check the main hub to be sure it isnt bent. Any slight bend at all and it becomes sticky and wont allow pitch changes. When you twist the blade grips with everything disconnected youll feel it sticking.

If you cant tell the difference between sticky blade grips or sticky main hub, then take hold of the piece that slides on the main hub and slide it by hand. If you feel ANY stick at all replace it.

You may also want to check the spindle to be sure its straight. Take off the lade grips and roll the spindle on something real flat like glass and look for a bad roll.