I don't get it. How can the previous batch POP the lid off if you have an airlock? I should just bubble like crazy. If its violent enough, it'll blow the water out of the air lock and you will have to put more in it. If it POPs off the lid, then either you are not securing the cover well enough, or the airlock is plugged. Did you check the airlock? Try sticking it in a running stream of water from your faucet, water should flow through easily. This might also explain why you don't see bublles and the CO2 is squeezing out the lid instead.
Just thoughts.