It might sound like a strange issue but it popped up in-game for me:

The town had both "defensive" upgrades that gave me extra Militamen for the towns defense; the enemy killed 2 of them (And I did as well, to double check) and then after that battle was concluded the town was attacked again in the same turn. For that second battle the Militamen did not spawn.

Edit: It should be noted that killing one Militamen is alright. Two creates the problem (More than two... Well, I'm not so sure)