I had a lot of issues with low temps. I tried various things - changing the intakes from pipe bushings to pipe nipples, changing the ash pan from the tabletop bbq to a commercial pizza pan. Then I found the answer (for my problem anyway - humidity.
First burn I used a partial bag of charcoal - it had been open for a while. Second burn I used a full bag, but it had been in the garage (near the dryer vent) for 6 months. Third burn I bought a new bag from the store. First two burns had trouble maintaining 225 with 2 nipples and a valve open. Third had no problems with 2/3 valve and -0- nipples open.
Humidity in the charcoal must just eat btus as it evaporates, plus it probably slows combustion as well.
Last week I ran a hot burn. I ignited lots of the remaining coals from the 3rd burn with a weed burner then ran with 1 valve and 2 nipples open - no probably maintaining 350. We have had several rains, so I expect my charcoal was humid again. Perhaps if you get it hot enough then you can dry out the charcoal - not sure.