If that much lump or charcoal is burning down quickly, say less than 7-8 hours, you may have too much draft going out the top end and loosing all the heat.
Try removing the long riser on valve in the pic...that tall riser may be trying to reverse or equalize the flow...should be going in to feed the fire, but is just not moving at all???
If you change the above and still don't have better results, and if it's burning slow and steady, but temps won't come up with 3 - 3/4" intakes, the inlet draft may be rising up between the sides of the basket and the drum and not making it far enough in towards the fuel. If you have threaded inlets on the drum, you could screw in 4-6" nipples (depending on the gap from the basket to the drum) to carry the intake air closer to the fuel.
Sounds like a finicky UDS, but let us know what you try next and how it works out...I curious about this one now.
Eric