After reading 40 pages of a message on Flyer Talk, I found out that if you have preexisting reservations on UA that you made before you got your Global Entry card, you have to call to have your" known traveler" numbers added to those reservations as UA has removed the "add the number" feature for preexisting reservations. I guess from what I read that particular feature is to be added back later on this year.

So, I just spent an hour talking to an UA rep who had no idea what I was asking him to do. Finally we got it figured out and he added the numbers. (I think, lol)