Duck breast; Trouble crisping skin

Without knowing exactly what's going on (i.e. looking over your shoulder), I can only answer in the general (some of which I assume you know, but will add in anyway).

1. Water / Moisture is the enemy of browning. I usually use two temperatures when cooking duck, which may just be a more extreme version of what you're doing. I'll cook low and slow until I've nearly gotten the internal temp where I want it, then I will remove the duck, wipe the pan clean, and crank the heat. While the pan comes to temp (which takes about 5 minutes or so), the juices are redistributing in the meat, which helps keep it juicy. If there is any moisture on the skin, I pat it down with paper towels. Then I will reintroduce the duck to the pan (skin side down, of course).

2. Also, added weight can be quite helpful to force the not-flat breast to smoosh up against the flat pan more. If you've got a second cast iron, while the main pan is getting ripping hot, you can heat the other - not to get the same temp, just to get warm enough not to cool the duck. When the duck goes back in, the second pan goes on top to press down and help get added skin-to-hot-surface contact. (If you've got enough good heat protection, you can pressdown on the top pan, too. Obviously, be careful.) I find the dual-pan trick to be only of limited usefulness. No matter what I've done, I've never had a pan heavy enough to completely flatten the breast, so I usually let the top pan sit for a while, then I end up taking it off and turning the breasts to a side that missed some browning, and trying to get all the bits brown that I can by moving around in the pan. It sort of works.

That's kind of the downside with any frying pan method to get duck breasts (which ain't exactly pancake-shaped) to brown as nicely as an all around method (like baking) can do.