I'm working on a HP M3035 MFP. When scanning double-sided using the ADF it works fine if only a single sheet is fed. But if multiple sheets are in the feeder it fails to scan both sides of all sheets and ultimately prints fewer pages than were in the ADF (i.e. when feeding 3 double-sided pages only one or two get printed). No error message appears.
Mechanically everything in the ADF looks fine.

Any ideas?