Messages may not be sending for several common reasons, including:

• The recipient's phone is switched off or out of service, so the message can't be delivered immediately and may be retried later but can eventually fail if undeliverable for a long time.
• The recipient has blocked your number, preventing messages from being delivered without notification.
• Your phone or carrier has connectivity issues, such as weak signal, no network, or airplane mode is on.
• The message is flagged as spam by carriers or filtering systems, particularly if sent in bulk or with suspicious content.
• Technical problems with the messaging app, such as cache or software bugs, can prevent the message from sending.
• Using incorrect or invalid phone numbers.
• Attachments that are too large to be sent.

To fix this, check your phone's network connection and signal, confirm the correct recipient number, and restart your messaging app or phone. Also, ensure the recipient’s phone is turned on and reachable. If messages are still not going through, consider that you might have been blocked or your message might be flagged as spam, especially if you send many messages or use business SMS services. In some cases, carriers may be experiencing outages or restrictions that affect message delivery.