M&P Sport II Trigger Reset Failure

[BeachAR]

I have an issue with my M&P Sport II that has about 1200 round through it. This issue seems to have started when I changed from Federal rounds to IMI but I'm not sure if this has anything to do with the issue. The other day at the range I fired about 50 round, then after about 3 more rounds from a fresh mag the trigger did not reset. I cleared the unfired round and let go the charging handle and again the trigger did not reset. I removed and replaced the mag after clearing the round again, fired off 2 rounds and it happened again.
I am new to the sport of shooting and built my own 300 AAC Blackout a few months ago which was easy. I clean my rifle after every range day and I don't see any excessive wear on the BCG. The BCG will eject the spent round and pick up a new round with proper placement in the chamber, but the trigger just won't reset. Everything looks clean and lubed and I'm just not sure what to look at. I apologize if this is a duplicate thread as I tried searching for a similar issue in the archive to no avail.
This happened to me last month once and when I replaced the mag it seemed to clear and reset. Now, it didn't matter what I did.
I am using the following ammo,
IMI 5.56 55gr FMJ

Any suggestion on what I should look at or recommendations are greatly appreciated.

[WMG]

Is not resetting as in "following the bolt home" when charging the gun or not resetting as in "the dis-connector isn't releasing the hammer"?
When it does not reset , crack open the rifle and see if hammer is back or in forward position.

[SkyHawk]

When the 'trigger does not reset' next time, pivot the upper and see if the hammer is cocked. Also make sure your trigger spring is working and has enough power to return the trigger forward.

When this has happened, have you tried manually pushing the trigger forward?

Here is a very simple video showing how the AR trigger works.

https://youtube.com/watch?v=xh2FjzVVIZY

[BeachAR]

Thanks, Skyhawk. I haven't tried pushing the trigger forward when this happens as the thought didn't occur to me. The video did help. When I crack open the upper from the lower to clean after coming back from the range, I can hand push the hammer back where the shear and trigger will lock into place. I will take a flash light to the lower and see if there is any damage or excess grunge that I may have missed. I've had a couple of miss feeds but this was the result of me not doing a good job cleaning the carbon from the BCG. This occurred before I started having this issue with the trigger. Once it was cleaned and properly lubed the rifle fired fine for 100 rounds.