I've not sent you a message. Can you check your inbox and let me know what time you didn't receive it?
My now late mother once sent me message from her new smartphone a couple of years ago. It read "Just testing new phone, have you received this text?" You can imagine the carnage caused, and fun I had, when I replied "No".........
If you have a certain number of messages in your inbox and/or sent folder it won't let you send or receive any more until you've deleted some.