BBC News snubs Radio 4's Feedback again

Why won't BBC News put up anyone to appear on the BBC's Radio 4 Feedback programme and take a grilling from presenter Roger Bolton? Today Bolton said that no-one from BBC News would appear on Feedback to answer claims by listeners that news bulletins had plugged Panorama's "Baby P" investigation when it wasn't proper news but PR for the programme. It is the third snub for Feedback. BBC News refused to appear on Feedback to defend the BBC's coverage of the economic crisis and before that, John Humphry's robust interview with Alistair Darling. What are they afraid of?