Yeah, right. Could be one or more of what's been said.

I had kind of the same issue and soldered on real bullets to my charge lead and that did it. It was the cheap ass bird cage plug things that we not connecting well any more or something in their solder job was going south. Scratched my head for a while on that one

Point being it's probably a connection somewhere down the line. Balance lead? Balance board? Port that balance board plugs into? Hopefully not a lost balance connection in the pack.

Do your race packs balance charge correctly?

As asked, does your charger show per cell voltage? If too low try NiMh charge at 0.5a for a while and get them back to where they should be.

If the immediate fixes don't work, try new plugs for cheap, solder well. Still no?
Keep going down the line.