Cabbage first lays in covering leaves, as if in bins, supply of nutrients, which then will use when starting to tie a head. As a rule, the head starts to tie is about 8-coverts leaves. Some gardeners deliberately take out these large leaves, ruining the bins, but sprouts again vigorously begins to grow. And there is a struggle between the cabbage and the owner of the site, and the cabbages all not fastened.
When the cabbage is using all the nutrients from the covering of the leaves, they turn yellow and will disappear as unnecessary. To remove these leaves from cabbage is possible only in case, when they are fully yellow.
If cabbage is not enough water and feeding, they will not. You need to provide abundant watering and well fed, then soon things will get better.
The cause may also be too hot weather without rain, the soil dries. Again, we need to increase watering.
Cabbage may not be enough light. It is a photophilous plant, and in the shadow of the harvest to impossible.
When fed fertilizers, must strictly adhere to the number of insertion feeding, because it can also be the reason that your cabbage is not fastened.