When I switched everything to the s21, I made it my default but soon realized that I could not use it when all my messages would not send/receive. I think most of the unsent/received messages were part of my conversations with my co-workers in the 20+ chat groups. I would get notifications that some messages were not uploaded/downloaded and when I opened the app, I would see in individual conversations, messages that required me tap on to download the message that was sent to the group. Even still, those messages would not upload to my phone correctly.
The only way I could have them properly sent to my conversations is to switch to the Samsung messaging app, tap on them to download correctly and then go back to the Google message app to see that they were then correctly sorted out.
Keep in mind, it appears that this is only happening to my 20+ conversation on a consistent basis but individual/smaller conversations sporadically.
Any thoughts on how to fix this?