Troubleshooting tips if your SMS are not sending:

Messages will not send if the body exceed the limit of 1600 characters.

If you are using a trigger to send sms. Check the body of message. Also, check to see if there is a custom value of {{message.body}}

Example, if someone replies via email and there email exceed the limit of 1600 characters, the message will not send if you have the custom value {{message.body}}