There are a number of possible causes for this.
1. Check shellholder alignment with ram; If there is a burr or spilled powder or some other foreign debris that prevents the shellholder from slipping all the way into the ram, the primer guide (the spring-loaded steel cup) will hang up in the hole through the center of the shellholder. Clean the "T-slot" for the shellholder with an old toothbrush or something similar.
2. Smooth the sides of the primer arm; If there is a dent in the side of the primer arm, the metal around the dent will be pushed up making the primer arm a tight fit in the slot in the ram. Sanding the sides of the primer arm flat will solve this problem.
3. Smooth the slot cut into the ram; The same applies to the slot in the ram - a dent or burr will make the slot too narrow to allow the primer arm to slide up and down freely.
