The press acctually has more compound leverage than you need,( I take it you are priming on the press) Check the priming cup that is the slideing thing the primer goes into to be pushed into the case, a small cup will not hold a large primer very well, or push it in deep enough. For the 243 WSSM you should be useing large rifle primers, they are all a strandard hight and seat just below flush when seatted properly. If yopu have been useing these in an AR I am surprised you have not gotten some slam fires. Pull down any loaded rounds you have that have high primers (DO NOT TRY TO RESEAT PRIMERS IN LOADED CASES!!!) I cannot stress that enough. Pull the bullets, dump the powder. Once that is done you can try to reseat primers NOT BEFORE! or punch the used primers out and seat new ones. All I can think of is that you just did not push these in far enough with the press ram.
