Are you replying to messages sent to you? Are you choosing the contact from your contact list? Manually entering the number to send the text to?
Be sure you are using the full 10-digit phone number for your contacts.
What phone do you have? Do you receive any error message at all? Perhaps your number is being blocked?
Let's make sure those messages are sending smoothly as possible. Have you noticed that this is happening with certain contacts? How long has this been going on? Do you have a lot of older message threads saved? If so, try deleting older messages. Please share detail so we can assist.
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