Originally Posted by VFR

I believe the criteria is:

1. It must be a mainline flight that serves a meal.
2. It must not be on an A321neo aircraft (there are a few threads on this).
3. It must be to/from a full-service catering station, because stations without full-service catering tend to get a cold or shelf-stable meal in both directions.

My guess would be that (3) is what's affecting your flight, because when I look at menu.delta.com I see three cold options for the JFK-PUJ route.