I have a watch ultra, I've never had any issues sending or receiving text messages. Now for the past month or so I can no longer send messages. It says message sent, but the message doesn't go through.
Are you talking about sending responses to Facebook Messenger notifications on the watch? This is not a Samsung issue, it is a Messenger issue. They messed up their last update. You need to put in a report with Messenger. In the app, go to the side menu, hit the settings and scroll down to "report a technical problem". In the meantime, go to APK mirror and download an earlier version of Messenger (v 485.0.0.58.111) and take it off auto update. There is an ongoing thread here: https://us.community.samsung.com/t5/Galaxy-Watch/Unable-to-reply-to-Facebook-messenger-notifications...
