The most common reason is that mobile number you're sending to is outside North America. Edit the contact in your phone book, and edit the mobile number to include the international dialing code format, e.g. 0044 or +44.
For example, you have a contact called John Smith with a UK mobile number saved as 07771 123 123 who is not receiving your messages. Go to your iPhone's Contacts and change John Smith's mobile number to +447771 123 123 or 00447771 123 123. Then, start Flutter and try sending a message to John Smith again.
If you're in North America, make sure the message is successfully being sent (and you see the "Successfully Sent" box). If you have continued problems, please contact us - make sure to include your country and carrier/operator.