Ok. My elderflower champagne is in bottles (1 litre PET) and was primed with 12g of dextrose/bottle. It's been bottled now for a week and they've firmed up nicely, BUT.... when you open one you get a very slight fizz and that's it, flat. There is some sediment in the bottom so I'm guessing it's done all it's going to do. Is there anyway I can fire it up a bit? Add a bit more yeast and sugar perhaps?