Work through these in order — they cover almost every case:

1. Sender registration (US): texting US customers from a local US number requires A2P 10DLC business registration; unregistered traffic gets filtered by carriers. See "A2P 10DLC: registering to send business texts in the US".
2. Recipient opt-out: if the recipient ever replied STOP, carriers block further messages until they text START.
3. Content filtering: messages with link shorteners, all-caps marketing language, or many identical copies sent rapidly look like spam to carrier filters. Vary the wording and use full-domain links.
4. Destination rules: some countries restrict or block SMS from foreign or virtual numbers, and per-country rates apply — check "Calling & SMS rates" for the destination.
5. Landline recipient: a text to a landline cannot deliver.
6. Credits: outbound SMS beyond your plan's inclusion draws on credits — see "How do I add calling and SMS credits to my account?".

If none of these fit, contact support with the destination number and the time you sent the message; we can check the delivery status on our side.