One of the seals in the ChaosSanctuary must we the last seal to open. Otherwise Diablo won´t appear.
hmm i forgot the hellforge seal twice and had both times opened all the other seals (also all the seals in cs)
when i opened the hf seal, diablo appeared in the cs normally and i could finish the quest..was no problem with it.
once i forgot izual seal,and unfortunately opening it didn't help and diablo didn't appear,so i think you only have to open izual first,for the rest it doesn't matter in what sequence you open them...