A few variables here but it all leads to the bolt stop. Next time it hangs open, check it's position. Sounds like it is engaging for whatever reason. These bolt stops don't usually give you a problem in themselves. When you shoot, are you staying clear of it's manual activation? From what you posted, I'm going to assume that you did not drop the trigger housing. The fix shouuld be uncomplicated. Let us know how you make out.
Be Safe !!!!