Yes, the buffer retainer can definitely cause a no-fire once it comes loose and gets hung up in the trigger group. Definitely, it can cause a no-fire. You installed a single point attachment. When you removed the receiver extension to put that sling plate on, I'd betting that the receiver extension needed one more turn when you put it back on. That's enough, right there, to let the buffer retainer loose (one turn less than needed). I don't have pics, I haven't seen the mod of the single point adapter, but I'm betting that's what it is - it has happened to me. With one particular brand of AR-15 lower I built, I thought I had it right, and this exact thing happened on the range - mangled the poop out of the spring, too. I'd already built two others, already - sure enough, I checked them, and the receiver extensions needed one more turn. Now, when I build, I'll screw that receiver extension in until it won't turn anymore - it contacts the buffer retainer. From that point, I'll back it out just enough until it's lined up to mount the stock. FWIW, this can't happen with rifle stocks and rifle receiver extensions, only collapsible stocks and their receiver extensions.