After several months of less than satisfactory 'fizz' from our Gemini drinks
maker and a few attempts at fixing it, I've finally worked out what was
wrong. The central spike that injects CO2 into the water had come a little
loose. Not enough to notice, but enough to let gas escape and preventing it
from getting enough pressure to dissolve in the water. Tightened it up and
it now fizzes and farts properly.

It's worth documenting!