Spells: Failing to dispel dots

[Dullah]

Twice in the last week I've been dotted, and upon dispelling the dot at the top of my buffs, it only dispels my other buffs beneath the dot.

Oh, i believe both times it was with drifting death. Dispelling with nullify magic from book, pumice crystal and also using the cancel magic spell. None of the three work on this spell randomly.