If you are using the cheapest steel-based ammo and your 870 is one of the Express models which are known for rough chamber finishes - that explains your issue.
Try your gun with some top end target loads like Win AA or Rem STS and see if it continues. Also, get a large brush, wrap with 0000 steel wool and a little oil, chuck it into a cordless drill and polish the chamber. THAT should definitely solve the issue
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