IMO the big killer on this one is the leaks. If your cooking chamber is leaking it will be hard to regulate and maintain temperatures. Another thing is the charcoal grate. This sits too low and does not allow enough room for ash and airflow. You will lose airflow below the fire too quickly. If you don't have a basket or haven't raised the fire grate, use the grilling grates to build your fire. This will allow adequate airflow under the coals.
You can also use sand or lava rocks (cover with foil for easy cleanup) in the water pan to help even out the temperatures and return to temperature quicker if the chamber has been opened. I've found using this I don't need a brick in there. Should you feel you need water in there also, use an aluminum pan on a cooking rack.
Over the next few days I would cook some fatties, ABTs, or something quick and easy to try different things to see what will help you achieve the temperatures you're looking for. If you've got the capabilities, try to monitor the different grates to get to know the hotter areas of the cooker. This can be helpful down the road when you need to move food around to try and get a little more speed on a slow cooking item.
Once you've got it going, this cooker is pretty nice. Wed night thru to Thurs morning mine will be cooking a ham, pork shoulder, brisket, baby backs, ABTs, and some mac & cheese. I'm shooting for cooking it all in one smoker, but depending on the nature of smoking foods (it's done when it's done, not when the clock says it is), I might have to fire up a second smoker to ensure that everything is done on time.