Every once in a while when you "catch air" off the terrain while riding on the ground or fall a very short distance (falling animation) you become no longer able to jump on your mount (and therefore fly). Despawning and respawning the mount does not fix it. The only thing that seems to fix it is falling (any distance). Also I'd like to apologize for bug reporting the Dread Knight ability Ruin earlier; it in fact works fine. :P