Be sure you are using a roll crimp in the crimp groove of the bullet, in addition to sizing the new cases. One time I tried loading a few 45LC in new unsized nickle cases, and with a good roll crimp the bullets did not push into the case, but I could hold the case in one hand and turn the bullet with my fingers, so I knew there was insufficient grip on the bullet and I started sizing the new cases. Nowadays on my 45LC and 44 Spcl rounds I also use a Lee Factory Crimp Die, which resizes the finished round again at the same time it crimps. You didn't mention what type of bullets you are using (.452 size is fine BTW), but if you are using bullets meant for 45acp then they will not have a crimp groove and it becomes even more important to give it a good crimp that bites into the bullet.