I have this turret press. The problem you describe is caused by the arm and the slot not being clean--probably the primer detrius when you punch out a used primer. Are you cleaning your brass and making sure the media is all out of the cases?
start with a thorough cleaning of the primer arm slot in the ram and the primer arm. Pay close attention to the slot sides and the back of the arm. When there is grunge buildup on the back and / or sides of the arm, it inhibits the arm clean drop, making it possible for it to pop off the pin.
Then, do a complete cleaning of the shell holder. Be sure to get under the rim and inside the hole before you reinstall it. I use gun scrubber / brake cleaner for this, and the only lubrication that goes on is a tiny drop on the pin.
When I do reinstall the arm, I double-check for completely free operation--i.e., the arm should slide up and into the holder and back out cleanly as the press is cycled.
Personally, I find that when I am reloading used (handgun) brass, I may need to do this particular maintenance every 200 rounds or so.
Jim H.