Why didn't the Allied Powers bomb Auschwitz?

The reason I ask this is because I was talking to an elderly man -the other day- who was in RAF Bomber Command. He said the RAF could fly over Germany in daylight, from about 1945 onwards. He said they used the crematoria chimneys at Auschwitz as a navigation mark. He thought -when he looked at photographs- that it would have even been possible to bomb the crematoria without hitting the camp huts. He said he had no idea what they were, and just umed it was an industrial plant of some sort.