It takes several cooks and practice if you are trying to reach a certain temp, best thing to do is to find its sweet spot/where it likes to run and hang out. Be patient and catch the temp you trying reach on the way up. Once you make an adjustment wait until you see it settling out then make a other adjustment if you need to. Me and a friend built 2 UDS's side by side same design same everything. They will run for about 14 hrs at different temps. Just don't give up. It also could be the amount of hot coals you start with, try starting with 12 to 15 hot coals first. Everybody has a different amount they start out with. Keep trying you will figure it out.
My arsenal, Stumps Clone, Two Modified offsets, open pit with grate and rotisserie,UDS, and a free to me Weber One Touch Silver 18.5" kettle