"set" ?
Sounds to me like the hammer is not staying or getting cocked. The firing pin in an 870 is "passive", for want of a better word, and simple rides in its tunnel in the bolt. It is struck by the hammer when tripped.
I'd pull the trigger group (surprisingly often not done), soak the whole works in cleaner, swab, degunk, blow off, etc, and see what that accomplishes.
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