Unable to tighten chain using the bar retaining button. When turning the knob (J) the TANG (K) will move when it is off the guide bar but when it is placed on the guide bar and tension is applied the Tang will not move irregardless of how much the knob is turned to try and tighten the chain. The gear on the end of the shaft does not engage or slips under pressure. Please sen d the parts to repair this problem.