If you just returned, it could be the case that not all of your miles have posted yet. This happens, especially with partner flights or flight segments that start overseas. DL tends not to post all miles for a trip at the same time.

Check back by logging into your DL account again in about a week and if you haven't yet received the miles expected, people here shoulc be able to help you if you post your fare classes (the letter that begins the fare code, Y B M S H Q K L U T X V E for economy) and the operating and marketing carrier for each segment.