Necromancer. Trouble (The Elder Scrolls 5: Skyrim)

I play a necromancer. No more than two assistants are summoned (pets, skeletons, atronachs... call it what you want) sorcery is fully pumped. Although people on YouTube have 5-6 all sorts of atronachs, ghosts are called and everything is fine. With me, if you call a third assistant, then one of the first two dies. Maybe the game is bugged or what could it be?