I had a beautiful plant but absolutely no head formation.
You are in zone 5a, so I would assume you are growing in a greenhouse or possibly a cold frame.
Romanesco is not truly Broccoli and not truly Cauliflower.
It is very temperamental to temperature and needs consistent water and highly organic soil to thrive.
It requires a slightly shaded location with alkaline soil and plenty of room to spread it's roots to spread.
Northern regions should plant in spring right after danger of frost has passed.