Can't Reply to Group Text

Recently, I have been having trouble replying to one particular group text. There are 4 of us in the text group. At some point (still not sure what triggers this), the "imessage" box at the bottom of the text disappears. So I have no way to enter a comment into the text chain. By accident, I found out if I start a completely new group text with the same people & send a comment in that new group text, I can then go back to the group text that was not working and the "imessage" box will have reappeared and I can then join the text again. It doesn't seem to be happening with other group texts, just this one particular group. Anyone experience this?