I had the same thing happen to me recently. I used some Northwest WorldPerks miles to get a ticket with Delta and even though I entered my confirmation number, etc. on the OLCI page it would not work. It would not even work at the kiosk either. The agent who issued the boarding pass said it had to be an e-ticket on DL "ticket stock" for it to work.

(Incidentally, this same agent "found" my itinerary, then realized I was travelling on an award ticket on an overbooked flight and then "had trouble" finding the itinerary, then "found" it again when he realized I was a SkyTeam elite plus member and would not take his shenanigans without a fuss.)