Make sure you have them set up for your flight. See this article for more information. Reminder: You must manually add a subscriber to each upcoming flight.
The number must be entered in international format (starting with a + sign). Here is a list of codes by country: https://countrycode.org/.
Check your provider's settings. In some countries, carriers may block such text messages. In this case, you need to contact your service provider to whitelist our text messages.