Common causes:

• No balance – incoming texts are only stored when your SMS balance covers one message; otherwise they are discarded. Top up and ask the customer to resend.
• Number mismatch – the reply is filed under Users not found if the sender's number does not match the phone on any customer account.
• Wrong number – the customer replied to a number that is not one of your Sprwt numbers.
• For US/Canadian accounts, replies containing only opt-out keywords (STOP etc.) are answered automatically by the carrier-level service.

Check History > Received for the complete inbound log.