I've had this happen to me when I got rebooked due to a delay. If I recall correctly, I submitted this form: https://www.delta.com/contactus/page...=mileagerefund

and explained that I did not receive credit because I was rebooked due to a delay. It took a week or two, but I did get the credit. I was on DL metal, so it may have been easier for them to research versus your experience.