There are several common reasons why a Delta flight might be ineligible for SkyMiles accrual. Most frequently, it is due to the fare class purchased; certain deeply discounted promotional fares, group travel rates, or "Basic Economy" tickets (depending on the specific program rules at the time of booking) may not earn miles. Additionally, Award Travel tickets—those booked using SkyMiles—do not earn additional miles on the flight itself. If the flight was a codeshare operated by a partner airline that is not a SkyTeam member, or if the ticket was not issued under the "DL" flight code, it may not qualify. Another possibility is that the ticket was purchased through a specialized consolidator or as part of a highly discounted vacation package that explicitly excludes mileage credit. It is also vital to check if your SkyMiles number was correctly entered into the reservation before departure; if it wasn't, you can usually request "missing credit" up to nine months after the flight, provided the fare was eligible. Lastly, if you are already earning miles for the same flight in another airline's loyalty program, you cannot "double dip" and earn SkyMiles for it as well.