What were they bottled in (glass or PET)? As Bum says, warm them up and see what happens before you do anything else.
Have you seen an improvement in carbonation since switching to white sugar? If so, are there any other differences between these later batches and the flat ones (e.g. where they're stored, type of bottle)?
If a few weeks at warmer temperatures doesn't do anything, then probably you either didn't get a good seal when your bottled, or the yeast is dead (very unlikely), or the carbonation drops didn't contain sugar (unheard of
). If you really want to try adding more sugar, do one or two bottles only at first, and keep them somewhere as bomb-proof as you can in case they explode. I don't know exactly how you'd go about opening a bottle that you thought might explode, but I'd suggest
very carefully would be a good starting point
. This assumes glass bottles. Exploding PET bottles will make a mess but are unlikely to kill you.