OK, tried a dry run on my new UDS last night and had a similar problem.
Got a full chimney flamin hot and dropped it on top of the charcoal basket. Basket had 2-3" of RO lump and 4 chunks of hickory.
Try about 15 briquettes and let it come up to temp slowly!
Lowered the basket into the drum and let it burn for 10 minutes or so with all intakes wide open and the lid off. Then I closed 3 of the intakes and put the lid on the drum.
You closed it up to tight, need at least 2 open at a 1/2" to get enough air.
When I went outside to check it 30 minutes later it seemed to have lost a fair amount of heat. Four and a half hours later the drum was cold and the fire was out. 85% of the lump and hickory was un-burned.
I am thinking the fire choked from lack of oxygen. But would love to hear others thoughts. My build is fairly classic. Lid has 8 1/2" holes. All were open. The pipe nipples are about 2" off the bottom of the drum. There are 5 nipples (each is a 1/2" nipple) and they are 4 inch long. The basket rests on top of the nipples and is constructed of a weber charcoal grate wrapped in expanded steel. The drum was completely empty with no airflow blockage.
Help! I want to cook some fatties tonight.