I think you will find it is down to wear on the cocking linkage joins, which cause slight movement on the pins. This then causes the cocking shoe to bind on its travel. If you really want it fixed, you will have to work out which joints are worn. If you can, find a new cocking linkage. I think you can remedy it by fitting slightly oversized pins in the linkage,to take up the slack, but I have never done it.
Lakey