When the warps on F25, 50, 76, 99 activate, you have a few minutes (2, i think) to claim your ashes.
After the time limit, you will be warped to the next map without obtaining Ashes of Darkness.
This is done so that when people leave the tower, they cannot receive duplicate ashes by retracing their path upwards.
People that charge ahead will always have their prize. On the other hand, classes like priest or bard are sometimes waiting for the rest of the party to pass through first... or, in my case, snacking and farting around during the run.
A good practice would be to ensure your entire party is active when defeating Moonlight Flower, Amon Ra, Baphomet, and the mobs on F99; Then afk on the next level, instead of sitting at the door.
(Sorry that i never actually mention it myself during runs... :>) I hope that clears things up.