WSM would NOT get temp up

I was smoking a turkey yesterday for some friends and wanted to cook between 300 and 325. I had about half a ring of partially burned K blue from the previous day's smoke, and when I went to dump the ash I realize that I must have had a smoldering coal because as it sat with the rest of the cooker off of it, about half the ring lit. I put a full chimney on top of that, for about a 3/4 ring of mostly lit and ashed over.

I let it go with all vents open, and put the 15 lb bird on when it was at 275 and rising.

That's the last time I saw 275 for hours. The temp at the dome dropped to around 200, 220ish at the grate. I cracked the door. No change. Dumped another chimney of lit. No change. Stirred the coals and added a half chimney of lit without the door. Got to 250 briefly and then fell back to 200ish. Dumped another chimney of lit. No change. This was like 2.5 or 3 hours. Eventually I had a heaping full ring of fully lit coal, no door, and I couldn't get above 250 until about 4 hours into the cook.

I had a foiled older shallow pan with a 12" foiled clay saucer in it, and a drip pan on the second rack under the bird with a can of broth in it. Well seasoned cooker, and I have gotten the temps high in the past, but lately it seems like it picks a temperature and just wants to stay there regardless of what I do.

Any thoughts? Anybody had this happen to them?

I didn't think so but I guess it was possible, since at least the stuff I used to start the cook (and was used for the previous cook, of which I still had a half ring) had been opened and in my performer bucket. It didn't feel damp and the stuff I reused was still warm in the cooker but the day before I did have a slight problem getting it really much above 220 for my shoulder cook.

I did open a new back, so I'll have to see if my next cook has the same issue.

Can you feel it when its damp? Or can it take up some moisture and you wouldn't really even know?

Thanks for the response. Anybody else have thoughts?

I had the door upside down and ajar, and then ultimately took the door completely off. The coals were glowing the whole time and I must have added 3 full lit chimneys during the cook.

I was dealing with temps in the mid/upper 40s, some wind but not a ton. I put up some wind screens to counter that effect somewhat.

Maybe it was the partially burned coal. I did have that thing overflowing by the end, glowing hot, and did not get up to 300.

Water pan was empty and foiled. I had a foiled saucer inside it and one 12oz can of broth in an empty disposable aluminum roaster on the second rack.

I checked my temp at the dome with a thermometer in the vent and with an oven thermometer. Either one might have a few degrees of play, but not 50-100. The slow rate of bird cooking as checked with my thermapen confirmed that the temps were likely right about what I was reading (if they'd been off and way low, bird would have been done much sooner).

Ultimately, I finished it off in the gasser. People still said best they'd ever had. I told them they should have tried the one I made on Thanksgiving itself. That cooked around 275-300 and was really good.

Thanks for the suggestions. I think the charcoal, either damp or 1/2 used, must have been the culprit from which additional lit chimneys were unable to overcome.