In my world there is no iron golem spawning

I play Minecraft pe for a long time, i try to make a iron golem farm after nearly 9-10 try i notice that there is no iron golem spawning but cats are then I search on Google about it I get that cats and iron golem spawning mechanics are same but in my world no iron golem spawning ,then I created a creative world and try on it but no iron golem spawning help me