Are you tweeting @Delta or @DeltaAssist? Make sure it's @DeltaAssist

Some people have said that they have asked @DeltaAssist to follow them just in case something comes up on a trip. If they are not following you and you find yourself needing them, simply send a public tweet to @DeltaAssist with a general message (something along the lines of "flight canceled, need help booking an alternate, follow for DM")

Also make sure that your Twitter is public and not private. Private tweets, even those with @DeltaAssist, won't be seen by them unless they are already following you.