[Jon]

Also you need to remember that all the NCO trained gunners were in the rear of the B17 and B24 apart, from the top gunner. They had all been taught 100% day in day out how to use the weapons where as the nose weapons on US Bombers were usually operated by Officers who were trained mainly in other roles with gunnery as a small part of the training eg: the bomb aimer.
However the slow closing speed was the main reason, attacking a US formation from the rear certainly exposed the Luftwaffe pilot to more chance of getting hit than a swift head on attack, breaking under the formation and away if Mustangs were about.