Keeping the bubbles in sparkling gelatin desserts
As Chris says, its to do with the setting time of the gelatine vs the bubbly.

Ensure all of your containers are chilled (maybe even frozen? - I put mine in 3/4 hours before making the actual dish) before you add the (chilled) champagne and get it into the bottom of the fridge as quick as you can.

I suspect though cannot prove also that a smaller container (I use wine glasses or similar) will work better than one large dish (trifle sized etc).