I've owned my Buckmark for many years and have enjoyed this 22 cal target pistol. But recently I have had a real problem arming this gun; it is incredibly hard to put back the receiver to cock the hammer. On several occasions I have taken the gun apart, made sure it is clean and oiled, and then have no problem with it the first time I go to pull back the receiver, only to have it immediately become almost impossible to do again. Any ideas on what may be happening?