As phazonfreak said, check your targets for resistance or immunity to poison damage. Also, how many Death Shards do you have? Dirge of Ceresa isn't that good of a spell if you don't have a fair number of Death Shards boosting it.
Alternatively the resist chance is that high that the spell is a total waste of mana. Does not even hurt very lowly creatures at all.
Resistance chances do not vary with the spell being cast, only with the target spell resistance and caster spell mastery. Moreover, even if the spell does get resisted, you will know because floating text will pop up saying "resisted" over each creature that successfully resisted the spell; spell resistance checks are not made at any time after the initial application of the effect, though any resistance or defense against the damage type used by the spell (poison, for Dirge of Ceresa) will continue to be applied each round of the spell duration. If you have a single-target resistible spell like Curse or Blind, you can choose that and hover over each potential target to get an idea of whether or not your caster can expect to affect that target.
Regardless, a level 12 caster, especially a level 12 Mage caster who went in for at least one or two of the Prodigy traits, should not have significant issues making spells stick to most low-level targets. A level 12 caster who didn't pick up the Prodigy traits might have issues affecting low-level targets that have some spell resistance traits, however, and all caster types will have issues making spells work against targets that are highly resistant or immune to the damage type used by a particular spell even if they have no trouble making the spell stick to the target.
