Torpor Stacking Issue
 

It appears that certain effects prevent torpor from landing. Tonight in a PvP happening, having Insidious Decay and Malo (not sure exactly the mechanism) on me prevented me from torporing. In a duel a few weeks ago, some necro dot prevented me from torporing myself. I asked around and supposedly this is not supposed to be the case.

I tried to replicate this behavior by debuffing myself and trying to torpor, but I was unable to with Malo and Insidious Decay (it always landed). At the time of torpor not landing on myself, I had a lot of buffs but I am pretty certain (both tonight and against the necro) that I had open slots.

Test it again, then watch your health. See if the HoT is actually ticcing and giving you HPs, and the buff is simply not displaying.

I seem to recall that same issue occuring with Regrowth vs DoTs.

Yeah I've noticed that before - this was not such a situation. I received a "Your spell did not take hold." message and received no heals.

Were you snared? My best guess would be that Torpor can't stack with a stronger snare effect due to its own lesser snare.