Not showing when Delivered, but message go through and are being "Read"

My messages are going through and showing "Read", but won't tell me if they've been "Delivered". I don't know whether to leave it alone or to double tap and "Send As Text Message". It's only happening with one conversation too. Can anyone help?

It's been happening for a week or so and I've tried resetting, just now updated to iOS 9.0.2, and about to try resetting iMessage.

I can't find a solution to fix it. Can anyone help, please?