Old Nov 2, 15, 8:59 pm
Join Date: Mar 2008
Location: SEA/YYZ
Posts: 1,561
I don't think it has to do with the fact you're in the middle of the trip, you're in Mexico, etc.

I've encountered this before where the flight is under airport control and you can't make any seating changes until check-in, although it hasn't happened in the last year or so for me. How close are you to departure? If it's within a couple days, this could be it.

I may try changing seats on the mobile website or app if you've got that.
United757 is offline