What's keeping someone?
What is delaying someone? (The someone is replaced by a person's name or a pronoun.) Bob: Wasn't Mary supposed to be here? Bill: I thought so. Bob: Well, what's keeping her? Bill: How should I know? Bill: I've been waiting here for an hour for Sally. Sue: What's keeping her?
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.