My cabbage heads are leafy, but not firm and solid. Are they missing some kind of nutrient?
Why Won’t My Cabbage Heads Get Solid and Firm?
There are some varieties of cabbage that don't head. If you're sure you have a type that is supposed to form a head, there are a number of possibilities why it may not. This article will give you some clues: https://www.gardeningknowhow.com/edible/vegetables/cabbage/cabbage-head-formation.htm