If you have a high level conjuror and you've been summoning two things at a time for a while, and lately they would sometimes appear, or one would appear, or rarely both would appear, this is for you.
It's the atronach perk! I reloaded a ton of saved files to narrow it down. It "absorbs" the attempt at summoning a pet 30% of the time and instead refills your mana bar (which is never what you want at the time your summoning a pet). Just posting this to hopefully help someone avoid the frustrations I went through. If it's already been posted somewhere else I wasn't able to find it myself.

Good post, I haven't had this problem yet as I'm not high enough level but I am aiming for both of those perks so it will be good to keep this in mind once I have them both.