Here it is:
It has a Weber lid with the standard 4-hole exhaust that I run full open. Your four exhaust ports are probably equivalent.
However I have six 1' diameter intake holes that I cover with magnets.
I start out with all of them open, then gradually close them off as I reach my target temp.
Your intakes have the added drag of the pipes and valves.
It is harder for a smoker to draw air from a raised inlet, rather than from the bottom.
You need a lot of air flow to get the temps up initially.
I believe you need to increase your air supply. Add two to four more inlets at the bottom that you can close off with caps as it comes up to temp. You can start by drilling two 1" holes that you can cover with refrigerator magnets, and if that seems like not enough, you can add two more. when you get it working, you can add pipe nipples that you can cap off. I personally just keep using the refrigerator magnets; cheap and simple.
Another factor is how you start your fire.
I start with 5 hot coals (ashed over in the chimney starter) that I place in the coal basket evenly spaced.
From that humble beginning, I am up to temp in about 20 min.
I hope this helps.