Arsenal Control - Tranquilizer damage

[Blueridger]

Sometimes, tranquilizer does not deal toxic damage when it hits a target that is already asleep. I am not sure if this is intended, but the power description does not read that it is.

[ZorkNemesis]

The damage is an independent hit check.  The dart will always hit and apply sleep (sleep powers are autohit in PvE) but the damage will still need an accuracy check.  Entirely possible you're simply missing the damage roll.  Check the Hit Rolls in the combat log and see what's happening.