Can one of you AR smarty pants help me diagnose and fix a problem please?

My go-to gun has been short stroking like crazy. The timing was confusing,,,it's my 14.5 BCM that I've had on a Smith and Wesson lower that's run like a boss. Figure I have soemthing like 2-3k flawless rounds down it. Got a BCM lower a couple months ago, had put a few rounds down to function check with no problem...then went to do some real shooting and it turned into a single shot rifle! Either failed to pick up another round or failed to feed the following round and banged up the case good in a jam. Tried multiple mags and several types of ammo and no love. Somehow had the gun very dry and wondered if that was it or the buffer spring/weight. I cleaned the heck out of it, lubed it and swapped back to my previous spring and tube...shot like 10 rounds and it functioned. Back to the run and gun range again Friday and it's back to being a single shot! I swapped lowers on another gun and still no joy. Still the same deal...failing to pick up a new round or failing to feed properly. Tried multiple mags.ammo again. I checked some single round shots and it did lock back open. I checked the bolt carrier key and it's staked and I feel no wobble. Shot some 10w-40 down it and see no leakage. Rings look good and have sufficient tension to hold its' weight. I'm pretty sure I swapped BCGs at one point when I was checking out the buffers and no love there.

I'm getting tapped out for ideas! This is my favorite AR and having it down sucks. Am I down to the gas tube and block at this point? If so...can you tell me what I'm looking for?

Help!!