SMS Marketing requires users to opt-in to receive messages and in many cases requires them to initially request the messages from their mobile number. If you're having issues delivering messages to people, follow these steps to troubleshoot:
Make sure they are set to Opt-In for SMS. You can check this on their CRM profile
If they're still not receiving messages, this may be due to them opting out previously. In this case, if they simply reply Start to your number, this will then re-activate their account.