Instead of forming heads, my cabbage plants keep growing as just broad leaf type plants. Why?? And if they don't form heads, can I just harvest the leaves like I do my spinach and use the leaves like I would if I broke them off a head and either make coleslaw or cook them with ham like you normally would?
...and yes, you can harvest the leaves. Bear in mind they could be tough and strong tasting. I think I would try a small amount before making a large recipe that's less than desireable.
Hopefully, this article will help: https://www.gardeningknowhow.com/edible/vegetables/cabbage/cabbage-head-formation.htm