Confused

For the first time this year I’ve had a net tunnel over half my brassica bed. They’ve all grown twice the height of the ones not covered. But none of the cabbages have hearted up. Have I used the wrong sort of netting? It was just some fabric I had in my sowing box. A stretchy fine hold netting. Would the colour of the fabric make a difference to what heat was produced within the tunnel or something. 
I don’t think they’re going to do much now, even though they look very healthy.