Skeleton / Golem Summoning & tactics Options

[tempest drake]

I am curious, and those it might not be relevant, due to no one almost never using the actual summon cards for their intended purposes(i think) But how often will skeleton / golem be summoned when the skill is activated? because I am curious as to whether or not the summon is either random or merely requires an open space for them to actually appear

I ask this due to noticing that most of the time, when i end up fighting against either a large group of enemies (like in BF-13-1) or against the plus shaped unions, even if i activate doll craft/necro gate multiple times and even deplete several enemies morale to 0, i won't get the summon, but when i fight against gulcasa and manage to activate it, it always takes(usually close to the third or fourth union member....for some reason) so it kinda made me think that there could possibly be something to actually summoning a monster , but i need some sort of confirmation first >.<