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Probably a burr or not a well defined corner on the material/metal. After I installed my new trigger, I carried just my lower around (inside my house) for a few days after work and kept cocking the hammer and squeezing the trigger, letting the hammer hit into my palm. I dont know if this will help you, but its what I did. I didnt have any hang up issues or anything, I was told by a gunsmith doing this might be a good idea.
Sorry, thats all I got.
I would see about disassembling the LPK and check for burrs on the disconnector. I would also look to see if the disconnector spring is set in the trigger correctly. Sometimes they do not get seated all the way down properly.
I had this happen with a bad hammer retaining pin/spring (pin that is inside the hammer which lays in the groove on your hammer pin). Pop out the trigger and hammer and see if anything looks out of place or falls apart.