This is not an unusual issue to have. Actually, I'd be willing to bet damn near everyone has experienced this at some time or another. Going to the Forster Dies, which I did also, will make ya a happy guy. Did me. So only knowing what we know I'd say right outta the gate it's "springback" or lack there of. But it could be also your technique throwing the handle and/or also lube or lack of. So many things can add to this so here's just a quick way to see.
1. If you don't say use a Q tip and lube for the inside of the neck, try it. You can use case lube or dip your necks in Dry Neck Lube (Mica). I do both, either lube or mica, don't ask me why.
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2. When FL sizing try what I do with my cases sizing and seating bullets. Pull your handle all the way down consistently for each case. Run the handle up and turn your case one half turn and run it up into the Die again. Do 10 or so and measure them and see if you're all over the place or you're good to go. When seating bullets I turn my case 1/3 turn 3 times with some dies. My runout is .002 or less doing this. I haven't had to do this with any of my Forster Dies, only my RCBS.
Your cheap fix I bet will be just turning your cases 1/2 a turn and running them as close to the same pressure on the ram. Give it a go. Would love to hear it fixed your issue. And yes, this is the quick n easy guess/answer without going into all the how many times fired, are they the same headstamp, do you anneal, what's your favorite color, blah blah blah. Give it a go.