If the gun is Boss then there are two or three things to consider .Firstly it is the spindle that is worn or has "twisted" with years of use . The simple remedy is to heat it up and set it back ,then re harden and temper .It could also be that it has worn both in the body and on the spindle cam ware it comes to a stop . Again the simple answer is to build up the leading edge. add to this possible ware on the bolt "stop" ,the bit that stops it going in too far.
Without seeing it all is educated guess work ,but those are the points I would look at first . This will entail stripping the gun and checking the position of lever and spindle both with and without the bolt in . Bite is another matter .
