Are they under nets? Three main common causes are caterpillars, snails/slugs, or pigeons.
I don't have cabbages on the go yet but I put a net over me spuds, carrots, beets and parsnips and the snails get trapped so don't ravage them. We have 2 wood pigeons that snoop about the net keeps them off as well.
I got my net from B&Q and use stones to weigh them at the edge I have put sticks around the veg with small plastic bottles on the top of them. This holds the net higher above the plants.