I already paid - why is a side still locked?

Because buying coverage and extending it are two different things. Your first purchase opens a plan with room for a number of relatives; a top-up adds more relatives to the plan you already hold — it extends it, it does not start a new one.

If you hold a plan and still see locked sides, you have used every place in it. Either release someone to move a place, or top up to add more relatives to the same plan.