Thank you - are you using the stock messages app or are you using Android Messages?
I'm going to shoot you a PM so that I can get some account details for you and make sure there aren't any short codes being blocked that would be causing issues in responding.
I'm experiencing the same issue on my Galaxy S7. I never used to have this issue and only noticed it recently. A few months ago I reset my phone to factory settings. Any ideas on what I can do?
If you can navigate to the store and download Samsung Messages (this is free). Once downloaded you can enable RCS through the app, and this will add you to the database. Try this. Let me know how it goes.
I believe I already have the orange Samsung messages as my default and no other messenger apps downloaded. I couldn't find it in the play store