If you can, measure the length of the case(s) with "no powder" -- are they on the short side? Also, check your powder drop/expander adjustment. A combination of short case or short stroke and marginal drop tube adjustment depth can result in the powder bar not moving far enough to pick up the powder charge. Had this happen loading .357s when a 38 special case got mixed in. As mentioned above, it's wise to look in "charged" case before seating the bullet -- I use a DeWalt snake light.

Paul

FWIW,

Paul