Last year was my first with a Hitzer 503, too. If you have good draft, then don't worry about whether the insert will work properly, it certainly will. I had a few moments if concern, too, but everything was perfectly fine after I got the hang if things.
First off, where do you attach your thermometer? The insert is double walled, so you will see temperatures lower than others report. For example, I have my thermometer attached about halfway between the hopper door and the right hand side of the stove top, and my temperature had never exceeded 300 degrees, and my 503 heats the entire house. Last night, it was 20 here, and my stove was 200 degrees and the coolest room in my 2200 sq. ft. living space was 75 degrees.
When I tend my fire, I do these things:
1. Open the ash pan door and upper damper until the fire perks up nicely, usually about 5 minutes max.
2. Shake the grates until I see a nice row of glowing coals starting to drop in both sides while shaking.
3. Open the hopper and fill it to the top. I carefully brush all crumbs off the flange before I put the hopper cover back so I get a good seal. You don't want a fire in the hopper from a loose seal, CO would escape.
4. Leave the ash pan and damper open until a nice covering of blue flames are dancing all over the fresh coal, usually about 3 to five minutes for me, then close the ash pan door and push the top damper in fully.
I suspect that your problem is that you are not filling the hopper. The 503 likes a deep bed of burning coal, and works like a champ when it has one. I had a few problems, too, until I learned that.
By the way, make certain to have working CO detectors installed properly. I've never had a problem with CO, but detectors are a necessity.
Best if luck with your new insert. I love mine.