check the properties of the snap dragon and change all relevant properties of you unit to match its. Also, if this is a custom slow poison, make sure that evrything except for the art section is the same as the true slow poison.
Unless I'm mistaken, you're trying to add a spell to the wrong place.
Abilities - Default Active Ability isn't where you add new spells. It's where you specify which of the unit's abilities are set to autocast when they spawn. For example, the Priest has the Heal spell on Default Active, so that spell is set to autocast whenever you make one.
You have to add the spell to the Abilities - Normal section.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.