ToME: the Tales of Maj'Eyal

It happens rarely but every once in a while, the summon ghouls "button" is faded out as if I were out of mana or similar. If I add another talent point to it, it reactivates.

It most often happens to start a game.

Ghouls summon every certain amount of turns automatically while you're in combat. You only need to summon the first one, for it to get started. After that they'll summon themselves when enough turns have passed, if you have mana and souls, without taking a turn.

I just realized the answer is that I'm an idiot...

No further explanation, ha ha.