I'll throw this into the mix
Yrs ago I was shooting 8x57s out at the gravel pit
I had the 1 and only squib load I've ever had in all the yrs I've been rolling my own(knock on wood). The primer went off and but no bang. Bullet was stuck about 2 inches past the throat in the barrel,and of course, we were done with that rifle for the day. I think I was running around 46gr of IMR4064 in that load, and as soon as I opened the bolt, there most of the powder was. Some of it you could tell tried to light, but it was rather "clumpy". Got her home, drove the bullet out with a piece of 1/4 round aluminum, then proceeded to get the collet puller out and start pulling bullets. In none of my other cases did the powder come out clumpy. Twas all loose like I just poured it from the measure. O boy did I go round and round trying to figure out what happened. O I figured it out, or at least what I think happened. At the old place I had a basement that I loaded in. Was down there doing a session and all of a sudden, a drop of water hit the bench. Looked up, and sure enough, there were water pipes right over top of the bench. Of course, twas a bit warm that day and the cold pipe was sweating a bit. Nearest I can figure is I had a similar situation the day I loaded that ammo and maybe, just maybe, when I had all the charged cases in the loading block, one of the drops went smack into the case. Dunno for sure, but it sure seemed plausible.
So if you case was charged and maybe contaminated, you'd find propellant in the guns action. It obviously didn't burn, so there should have been some in there. If not, you may have double checked but still missed that particular case. Or your measure either threw a very very light charge or no charge at all