Tom,
The biggest risk from the DRDT-2 tool is
UNDER dimpling. Your picture isn't real crisp, but if I'm seeing shallow depressions around each rivet, it's definitely under-dimpled. They really show up if the light is reflecting just right. I had this same problem early on with mine. My tech counselor at the time showed me his C-frame and how he used it, whacking firmly twice on each dimple, with absolutely perfect results. I heard the famous quote "you gotta beat that aluminum into submission". I realized I was being too timid when it came to dimpling.
I was later able to duplicate his nice dimples on my DRDT-2 by adjusting the ram downward and locking it in a position so that the dies make contact with each other about 2/3 of the way through the handle stroke. This is with NO aircraft parts in there; just the dies. When I'm dimpling, I pull the handle down once to form the dimple, and then I slam it down hard a second time on each one to really "coin" the dimple. The force is strong enough that you can actually see the big metal beams on the tool separate and deform slightly with each one. Try it on some scrap. You should see a big difference. I have more discussion and pictures on my web site here:
Dimpling with the DRDT-2
I also agree with a previous post about quality dimple dies. I spent the money on good spring-back dimple dies. It's worth it. Good luck as you progress!