The creatures summoned by this spell and theme attempt to cast Improved Invisibility before initiating combat, but they seem to have some spell failure from armor. So often they'll try to cast that single spell multiple times in a row and just stand there ineffectively for a long time if they get unlucky.
It would be helpful to list the name of the summon.
Sorry! The name of the creature summoned is Shadow Slayer.
Not sure what the issue was, but I removed the II because they have 50% concealment anyway so hardly needed it.