standard operating procedure ex Dxb is to do a waitlist for j to f with miles on busy flights - if you have a mobile BP and refresh you could have had 30mims in the f lounge as the upgrade is done at t-60. Contrast this to an outstation where space available you can use miles at t-3.

upgrade availability is determined on loads and forecasts - the other flight you took on that day was probably quieter hence availability.