Text messages may not be delivered due to various reasons such as using the wrong type of phone number, being flagged as SPAM by the carrier, recipients opting out of messages, containing a link or risky shortened URL, sending too many messages, targeting unavailable numbers, network problems, illicit content, complaints from recipients, roaming issues, billing issues on the recipient's side, incorrect number input, and time-outs. Businesses should ensure they follow industry best practices to avoid blocks and improve message deliverability.