I have an older 870 Wingmaster, sometimes when the safety is pushed off & you try to fire the gun the trigger does not move. I am fairly certain that a little piece of something is caught up in the safety mechanism. In order to take apart the safety there is a pin that has to be driven out, it looks like this pin has some sort of Loctite or other glue type material on it, does heat loosen this material up?
I never really noticed anything on any of the ones I've had apart.(I shoot left handed and change a couple to left hand safetys) I always just took a punch and knocked the pin out. But mine were older 870/11-87's so maybe they started adding some adhesive.
I've converted a couple of these guns over to left hand safeties as well. Best I can remember is it takes a punch and light tap with a hammer.