Smoking failure - Beef Cheeks came out leathery and wouldn't get above 200F

Yesterday I tried smoking beef cheeks for the first time in my Weber Searwood 600. I started smoking them at 225. After three hours they were up to about 160 and I put them in a foil tray with some beef stock, covered with foil and increased the smoker temp to 250.

After another three hours the meat had only gotten up to 175. I increased the smoker temp to 275. I had temperature probes in three cheeks. The temperature went up but stalled at between 185 and 205 depending on the cheek. I kept the cheeks in for a total of 14 hours until I gave up at 3am. They are rather tough and a bit like beef jerky, but tasty.

Why would the meat stall and never go to a higher temperature? I was using probes from my Thermoworks Signals and from my Weber Searwood which gave consistent temperatures. And I was using the oven probe from the Signals which should that the heat was where it should have been set. So I am confident that the smoker was the desired temperature.

For the first six hours I was also smoking a pork butt and it came out perfectly.

Why did I have a fail like this? Why wouldn't the meat keep cooking and go above 200?