Found a pretty funny bug. I got the skill Vampire to increase my vampire production. But i didnt upgrade it right away. I had it at level 0 for a minute (dont ask me why). I was out of combat and had my focus on vampires, although it always stayed at 0%.
I think you do the normalProductionRate x skillLevelEffect. which is incorrect. You should be doing normalProductionRate x (1 + skillLevelEffect) where skill level is a float and 1% is 0.01. or if skillLevelEffect is an integer you simply divide it by 100.0