Hello, I'm coming back to wine making after many years away. I'm using plastic demijohns for the first time. The wine is bubbling away but nothing is coming through the airlock....... How can this be? I've checked there are no leaks. I've even taken off the airlock and lid and put clingfim over the neck of the demijohn so there's no way any air/CO2 could get out and still nothing.... the wine continues to bubble and the clingfilm stays still. I was expecting a balloon to form....nothing. I can't understand how this can be happening. It CAN'T happen! Any ideas? Thanks!