Do any of the bottles have any sediment in them?? First thing I would do is check that, and GENTLY shake them to rouse the yeasties...and give them another week or so. If that doesn't do the trick, then I think your only option is the last choice you gave, open them all, add a neutral yeast, and recap, and PRAY. Not sure I would add more sugar tho, if the yeast didn't consume it the first time, it's still in there, adding more may cause an over carbonation problem, and potential bombs.
HTH
Let us know what you decided, and how it turns out. Good luck brotha!!

edit: One other thing to check, is make sure the bottles are getting capped correctly....they could be loosing CO2 that way.