Just today I was loading some 44mags for a friend, using his brass all sized and primed and belled. The seating/crimping die shaved a perfect circle around the jacketed bullet. I checked the bullet dia, the inside case dia, the whole gambit before I tumbled to the case mouths not being chamfered. Chamfering the case mouths ended the problem.
In the past I have found an occasional bullet oversized, obviously in the wrong boxes. Like a .311 in a box of .308's. I've never had a .311 anything so it happen on my bench. Only a couple times in 50 years, last Dec being the anniversary date of setting up the old Herter's Super Model 3 press and embarking on a lifetime of wonderment and fun. Check it out and see if chamfering doesn't solve your problem.
Regards,
Sweetwater