This is a more general Android thing, than a specific Pro1 question.
The delivery receipt is something the sending phone can request, but we have no guarantee that the recipient-system will actually send a receipt back.
I've been using "Textra" for years, that also exposes this, and on most SMS I get it. But on MMS it seems less stable. And if people are roaming on another net than their home net (e.g. during holidays) don't expect the receipt to be transported.
For the messages you already sent, you can only get the confirmation if it was requested at send.