if your brassicas are doing well in the same soil, it could be a PH problem. peppers (like tomatoes) need lots of water and Potassium (potash), high potash will will make brassicas bolt and seed very quickly. keep them seperate next time, and feed them weekly with high potash feed, they don`t need ALOT of water, but the watering must remain constant in regularity, that part is critical for good size fruit(s).
grow them in the same place and in the same way you`de grow Tomatoes, they can even share the same growbag!
they`re both of the nightshade family and alot closer rellated than a Potatoe is to the same genus.
regular water, high potash and a lower PH. and don`t start feeding them until the fruits set