RE: Is your Voltwatch 2 doing this??
If set to 4.8Vcc and checks out OK with a 4 cell battery, and when switched to 6Vcc setting and your 5 cell packs show a fail, I would be looking at my 5cell packs, as I would say they are bad. You don't say weather your packs are NiCd or NiMH, or how old they are. Old NiCd packs I would say that you have a bad cell.
Cheers