I have a older LWRC M6A2 10.5” upper I have on one of my SBR’s. A couple of years ago I installed a Slash/Heavy Buffers Anti-Tilt buffer you avoid carrier tilt, especially on these orders LWRC’s. Lately after firing the last shot the bolt does not want to stay open; manually the bolt catch does not want to hold open the bolt. I changed out bolt catch spring thinking it was the bolt catch spring was worn out. I tried another upper and a standard H2 buffer and spring and the bolt stays open on the last shot and bolt stays open when I manually engage the bolt catch. I am thinking it is the buffer spring that is underpowered now, so which one should I use to replace it the old one?

is the Slash buffer heavier than the buffer you took out?  sounds like it's marginally too heavy?  BCG not coming back far enough to engage catch, just borderline.  could also be you're shooting weaker ammo?  so not as much recoil energy, or gas energy?

going to a stronger spring will only make it worse, I think.

you could try one of those buffer tubes that has the extended lip, to help out with carrier tilt.  POF, or the new Aero enhanced buffer tube, probably some others.

Describe this more in more detail - could be the key. Bolt won't go back far enough to engage? If so, buffer is too long for your receiver extension, or there's something preventing it from going all the way back...

Will the bolt carrier group go back far enough for the catch to get in front of the bolt face?  I've run into at least 2 different situations where tinkering with BCG or stock components resulted in the bolt not being able to travel all the way to rear of the tube, so the bolt catch didn't have any way to engage.

Never had this problem on this lower before, and the gas key is not loose. I switched out lower receivers along with Heavy Buffer, and it looks like it locks back now on an empty magazine or last shot fired. . Have no clue, but maybe the lower receiver is the culprit. Thanks for all the advice And help guys.

I was referring to the ability to lock back manually being the key to diagnosing and solving the problem, not mentioning the carrier gas key as a possible source of the issue, which it shouldn't be. Sorry if it read ambiguously.

I finally found the culprit of the problem: the buffer spring. I changed out the buffer spring with a Wolfe Extra Power Spring and the problem was solved. Guys, thank you for all the advice