This usually happens when you make repeated payments in a short period of time. If earlier transactions are still pending and haven’t been finalized by the merchant, the funds from those payments are temporarily held. This means your available balance may appear lower, even though your total balance hasn’t changed.
Until the previous charges are finalized, new transactions may be declined due to insufficient available funds. How quickly a merchant finalizes a charge depends on their business and processing systems. Most merchants settle within a few days, but they have up to 30 days. If they don’t complete the transaction by then, the pending charge will be automatically cancelled and the held funds will return to your account.