I've seen just the opposite. In my experience (as an NWA-lover) the PFD was something that the NWA crews used any excuse (late catering, full flight) to avoid - even when they had 15 or 20 uninterrupted minutes to use to yap among themselves.
The Delta crews always were near 100% perfect with the PFD, and since the takeover I've had better luck even with the NWA crews - suggesting that their Delta trainers had made a point of stressing the importance of the PFD.
Maybe a Delta FA can let us know what policy, but in my flights lately it's been a consistent PFD on each one.
I am really surprised, and I don't think "it just isn't done anymore."
I would TTU with the date and a description or name of the FAs involved, because I think they were wrong, and out of line to push back on you for asking.