Often you will see one or two Air Atronachs and some Spellfiends at the places where there can be Frost, Flame, or Storm Atronachs, but there isn't a Frost, Flame, or Storm Atronach there, just a spot where there's mist, lightning, etc., coming from the ground.
A lot of players will run up to the group of mobs and immediately start burning them down, in which case the Frost, Flame, or Storm Atronach will not spawn in.
But if you just run up to the group and stand there for a second without attacking, the Frost, Flame, or Storm Atronach will appear, at which point you can begin fighting them.
This is another example of when running around and mindlessly burning down everything as fast as you can will actually make it take longer to earn an endeavor, while being a little more slow and thoughtful will help you destroy the required number of Atronachs more quickly.
Speed and power do not always equal efficiency.
I've fought mudcrabs more fearsome than me!