Block Spawn Mob One Time At Night

Hello, I am making a mod where I have a village in the nether, with four blocks in the center of the village. Those four blocks make a ticking noise during the day and stop during the night. I made an undead entity, and the idea is that each of the blocks will spawn one of them when they stop ticking so that the player will have to fight 4 at a time. The only problem is, I can't figure out how to make the blocks summon only one entity right when it turns to night. I have a procedure that sets the "Entity Spawn" variable to "True" every time the blocks make a ticking sound, and then, if it's night time and if that variable is "True", It spawns the entity at x y+2 z and then set "Entity Spawn" to "False"(This procedure is on block tick update FYI). Theoretically. But it doesn't spawn a single entity. Any ideas why? I've tried everything I can think of to try and fix this, please ask if you need any more details. Thank you!