How do you stop the airlock from filling up with the contents of demi john when making marrow / pumpkin rum?

I have been making Pumpkin rum (as marrows are now out of season) the sugar and yeast and spices were added to the hollowed out pumpkin just over a week ago and as the pumpkin had become soft i transferred the contents to the demi john and some scrapings of the inside of the pumpkin. it almost completely filled the demi john. now i find that instead of gases escaping via the airlock the gases are just pushing the contents of the demi john into the air lock.. i have emptied the air lock several times so i'm fairly certain that it isn't submerged in the liquid so i'm unsure why the gas isn't escaping properly!