What version are you playing, what height do you put the windmills at, is there space around the windmill, did you put a rotor in the windmill. which rotor did you put into there?
v.1.0.2.2 I've tried at multiple levels between 60-200, the only blocks occupied was underneath the windmill where the fibre cable was going straight down, Yes I did put rotors in them and tried every rotor to make sure it wasn't just that 1 rotor. I have also tried the different kinds of windmills as well.