Retrieval cues act as triggers to help us access a memory. When we make a new memory, we include certain information about the situation that serve as clues to access the memory. A Retrieval Cue Failure happens when you are unable to recall a memory because none of the clues are present to trigger it.
For example, when you go shopping, you are often attended to by a certain lady at the checkout counter. Each time she rings up your purchase, you exchange greetings and engage in small talk. Even from afar, you always recognize her in her uniform at her assigned counter. But when you run into her at a coffee shop in your neighborhood, you are unable to recognize her because she is out of her uniform, in a different setting, and is a customer at the shop instead of behind a counter. In this case, none of the retrieval cues that go along with your memory of the checkout lady are present, and this makes you unable to recognize her (She probably doesn't recognize you either!).