Android Messages Not Being Received - How to Fix
UPDATE: Verizon (my carrier) delivered an operating system (OS) update that fixes the underlying issue I was having receiving text messages. This video may still help, but check for OS updates if you're having this issue.
This video walks you through how to clear your Android "Messages" app's cache and data (specifically, on a Galaxy S21 Ultra). This app's cache and data getting out of hand is a common cause of text messages not being received.