I'm sorry if this is a duplicate, I thought I posted this, but can't find it now...
If I send a text message to my both of my parents, who both have Samsung devices, it will convert it to a chat rather than a MMS. This chat "resets" at the conclusion of the conversation. So the next time we send a message, it has no history of the previous conversation.
Switch your messaging app, either temporarily or permanently. I understand the Samsung app is having some issues. You are not the first one with this complaint.
I reported this about a week ago and "realaud" also responded to my post.
So far, it only has happened to one mass text conversation. One of them and I both have an IT background and we could not figure out what has happened. We deleted the conversation on all 3 phones and when attempting to recreate it, it only allows for a Group Chat, not a mass text.
We're guessing Samsung pushed something in a recent update that **bleep** it up, but no one from Samsung will chime in to explain what the **bleep** is going on.
Recommending switching to another messaging app is NOT a solution and just an "easy way out"...good troubleshooting is to determine why the problem exists and fix it.
