If you don't get a refund associated with your premium tax credit, it means that the correct amount of credit was applied to your premiums each month, so you are not due a refund to make up for getting cheated during each monthly premium payment.
Your credit is the amount that your insurance premium is reduced. For instance, if your insurance costs $1,000 per month, and you get a credit for $500, you would have to pay in $500 each month. It is not possible to get a credit that is more than what your insurance cost. Some people get a refund associated with their premium tax credit because it was more than what they received each month, but the total credit would never be more than your insurance cost.