There is another difficult to identify problem of the Remington 870. Sometimes, it is very difficult to to pull the forend after the shot. You need to pull it several times to extract the fired shotshell from the chamber. Polishing a chamber helps but it still can happen.
Remington 870 Police trigger group isn’t made of plastic as the one you have on your standard 870 Express. It is made of aluminum. And it also has some different parts like the carrier latch spring which can fix problem with fast shooting.
Failure to feed is a pretty rare problem which is also difficult to identify. I didn’t understand what happened first times when I experienced it. You feel that you hit something when you push forend forward trying to feed another round and when you check what happened you see a shotshell on a carrier and everything looks normal.