Try both primer arms in the press using the same shell holder. If only one sticks, then the problem is probably related to the primer arm assembly. If they both stick, then the problem could be the ram or the shell holder. Be sure to test this with a case in the shell holder, because in use the primer cup does not pass through the shell holder, just up to the case bottom.
If the primer arm cup hangs up in the shell holder, look for burrs in the center hole. You can also check to see if the priming pin has been bent somehow. If you lay it on a flat surface, you should be able to see if it is not parallel to the surface. If you can not straighten it, return it to the factory for replacement. Another cause is the primer cup has been bent and is no longer round so it sticks in the shell holder. Try to make it round again or return it to the factory.
