Brassicas

Hello, I am new to veg growing and would like to grow cabbage, Brussels sprouts etc. I have a heated propagator, a heated greenhouse and a cold frame. I sowed cabbage Hispi in the heated prop and as soon as they germinated moved them to the heated greenhouse. This was 4 weeks ago. I kept the soil moist by standing them on a watering tray with damp matting. They did not develop any true leaves, only the seedling leaves. Today they died. What did I do wrong. Should they have been in the cold frame not the greenhouse? Also the greenhouse is covered in bubble wrap so is this affecting the light?