messages on my iphone won't sync to my Macbook, although both use the same Apple ID password

text messages on my iPhone 11 have stopped syncing to my Macbook pro running Big Sur. No recent software changes. I've updated my Apple ID and password on both, and they are the same, but this doesnt help. I haven't found the help screens provided by Apple helpful either - they just tell me to do what I have already done. Maybe the solution is so obvious to the Apple experts that they can't imagine anyone needing help with it. Regardless, my messaging is not working the way it should. I'd appreciate any suggestions !