How did you pay for the ticket?

Assuming you paid with cash or Skymiles (including credits, etc), you should have received credit. Something doesn’t add up, but since you’ve resolved it using another program, there really isn’t anything to discuss.

FWIW, the flight was just 1 week ago. DL takes weeks to respond to missing credit, so you didn’t give them enough time to fix whatever caused your initial glitch.