Purchase 50 brand new Starline cases (or whatever brand you can get) and start over from square one . Reset all the dies carefully following the makers instructions .
If you are still having problems .... it's the Dies . Return To Maker.
Starting with new brass will eliminate one possibility that the brass is wonky .

I came across a batch of 303 British fired cases that were so oversized I had to partially resize them twice before getting them completely into a full length die ... the brass was so over worked after that ...one or two reloadings were all I could get before the cases started to develop signs of head separations .
If this is the case (pardon the bad pun) just buy a batch of nice new brass and your life will be better .

And do check the depth of the expander ball/ decapping pin unit, if too deep it will keep the case from going completely into the sizing die .
Gary

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NRA Member