Points of clarity, I am casting the spell in open relatively open hallways or rooms. The crosshairs are not red. I am holding the spell long enough before releasing it (i.e. the animation is complete and I've gathered all the energy into my hand). When it whiffs, the room turns purple-ish and I lose a small amount of magicka - but not as much as if I'd actually summoned the flame atronach - but nothing manifests. I am usually sneaking when this happens, but I'm not sure if that's the cause. And inevitably, I blow my cover and get mobbed without my trusty flame guardian backing my play.
What am I doing wrong? Thanks for your help!

) the Alteration spell absorption perks. Spell absorption can cancel your own summon spells.
