Hmm. It's not entirely clear what the problem you're having is, but hopefully this will help:
The two things most closely entangled with the Domain are the Growth and the Approach. If you've selected those, the Domains will be locked down to two options.
There are eight different possibilities, or three bits. The Approach is one bit: it determines whether the MC Throughline is in the top half (Situation and Activity) or the bottom half (Manipulation and Fixed Attitude). The Growth is another bit: it defines the relationship between the MC Throughline and the OS Throughline. (If the Growth is Stop, they'll both be in the top or bottom. If the Growth is Start, one will be on the top, and one will be on the bottom, adjacent to each other.) The last bit is your choice. If your throughlines are locked in like that, it may be that you've set the Growth to Stop and the Approach to Do-er.
If you've selected the Concern or Issue for a Throughline, you can't change the Domain. So if you've made the OS Concern Present, the Domain has to be in Situation. You can try to remove the Domain, but it won't let you change it to anything else, because Present is always a Situation Concern.
I hope that helps!