First thing you should do is contact NoDak and ask if they ever had this problem with that receiver, this is the first I heard of it happening with anything other then the two you mentioned.
Also check your disconnector, maybe it's an issue with the dis spring. Or maybe it's one of those defective disconnectors? The ones that had the spring hole made too deep, but I think that would of showed up a long time ago.
Another thing is to check the legs on the hammer spring where that contact the back of the trigger, if one or both of these legs get bent in they hit the side of the diconnector and this holds it back so it doesn't work the way it should. Check this first since you can do it by just removing the top cover.