Firebolt skill issue

The firebolt skill always dismisses my skeleton warriors.

That's because the Firebolt and Skeleton Warriors skills are provided by the weapons your character is using & not by skill gems.

So when you attack with Firebolt your character swaps to the staff to cast it, meaning they swap away from the scepter that was providing the summons; hence skeleton warriors despawn :(

It's a rather poor side-effect of how they've implemented these summons and weapon-swapping unfortunately.