I don't work for Spirit, my only experience with the jumpseat there is taking it, so I don't know the answer. I do know that when I've jumpseated on Spirit, they have made every effort to get me on.
An exceptional group of people over there - I suspect the only way you won't get on is if every seat is full.
I've called dispatch down in FL, and they have looked at the loads, and done what they can.
iaflyer