Mileage award availability

I'm currently booked with a 25K mileage award R/T BWI - MSP. Have been checking daily to see if any of the 45K miles FC seats open up, but no joy to date. Looks like there are a number of open FC seats. Does Delta typically open them up for the lower mileage awards as the flight gets closer (assuming they remain unfilled)?

I book this flight every year, and that has been my experience in the past, but I am not seeing the day to day changes in award availability that I remember from the past.

Thanks!