could be you have the wrong seating punch for the bullet design you have but most likely whats happening is your dies arent adjusted right and when the crimping is starting the bullet isnt seated to its final dept and when the case rolls into the bullet its being jammed into the case mouth. Put a case is your press with your seating die and adjust your crimp without a bullet in it. Then using another flared case ajust your bullet seating dept. Do it slowly though and dont push the handle the last little bit and crimp until you get the bullet to the right dept. Best way to avoid all of this is to seat your bullet in your seating die to the proper dept in one operation then back the seating stem off and ajust your die to just crimp or better yet but an extra seating die and pull the stem out and just use it to crimp. Some guys like the lee factory crimp dies for crimping but i prefer another seating die gutted. Factory crimp dies tend to iron out your case and you can loose case tension and if using cast bullet it can actually size your bullet down to a size to small.
