It took mine a while to start on the cabbage. I do remove the big loose leaves but they do the rest. Don't give up! Once one figures it out the rest will jump on the wagon .
I have a couple of small ropes tied to the wire. Then I poke a hole through the top end of the cabbage with a screw driver and attach it with a zip tie so that the cabbage is about head height for them. It may take a little while for them to get used to it but once they figure it out it is quite...
I can't keep mine in green cabbage. I hang them on the fencing and it keeps them occupied for hours. I have 20 chickens at the moment and some days it only takes one day for the cabbages to be down to the nubs !!