Certified GKH Gardening Expert
It may be that it is too warm or that you just need to give them some more time. There are a few pests that can cause this too. This article has more information on them:http://www.extension.umn.edu/gardeninfo/diagnostics/vegetable/cabbage/headnone.html
If you suspect that it may be heat, mulching well around the base of the plant can help.