I'm looking to use some of my M&M miles on UA to fly domestically in the U.S. Travel is about 1.5 weeks out from now and I can't seem to book the flight I want. Anyone have experience with UA opening up more award space closer to the departure date? I don't want to get shut out of everything, so I am wondering what the optimal timing for taking one's 2nd or 3rd choice flights when the first choice doesn't seem to want to open.

There are seats left on my first choice flights for purchase.

Thanks!