Burrs
The burrs that cause this problem can be near microscopic. Use a 10x jewler's loupe and inspect the edges. Stoning the mating surfaces can sometimes give rise to burrs so be careful.
Also, twirling a plug of 0000 steel woll in the closed cavity can help. Be sure to clean the grease out after this. Also, smoking with a butane lighter can help. Don't use a match as they can leave an oily residue.
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