can't chg Compressor dvd Field Dom setting

After not using Compressor with an Avid Quicktime (ntsc out) to make a dvd in a while, I ran into strobing / field dominance problem. I had used the default Best Quality 90 min forgetting what a wise man said on Cow "never let compressor figure out the field order" When it dawned on me to set Field Dominance in the Video Format tab to Bottom* - I couldn't do it, it was stuck on Automatic and grayed out. Clicking the little gear icon to the right didn't help. Luckily I stumbled on another older dvd setting where I somehow succeeded in changing Field Dominance to Bottom in the past, used that and problem solved.

Should this kind of thing come up again, what was I doing wrong? Why couldn't I change Field Dominance?

Thanks in advance,

Paul

*copied the default Best Quality 90 and tried to make the change on the copy