Or perhaps an even simpler cause, if I understand the problem correctly (pictures scanned into the old 'puter would help a lot).
Is the bullet tipping over as you run the ram up? I like to use the minimum mouth belling possible, which means that sometimes a short case doesn't get quite enough and it loses its grip on the bullet base before seating begins.
So, I often guide the case/bullet up into the die. Once in there, the bullet can't tip over and catch the case mouth. With my dies at least.
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