Credit score reduced due to missed payments even though payments were made on time

Do NOT purchase from this store! They will consider the payment of your instalment as a missed payment on the month on which you open an account with them. This will significantly reduce your credit score. I have made numerous attempts with the store to correct this but all they tell me are excuses. Even when I pay my instalments on the agreed date, they still counted it as a missed payment. Their reason for this, was because I paid on a Saturday