You may not have a perfect seal, so the gas is dissipating into the atmosphere. This is not a problem. The airlock serves only as a safety mechanism - it prevents buildup of gas, while also preventing anything getting into the container. An airlock is one of many ways to achieve this. You can put a bit of aluminium foil over the lid, a teatowel, anything that won't let dust or bugs through, basically.
Don't worry about the balloon, and never ever use the airlock (whatever type you use) as an indication of fermentation. Gas coming out can be caused by more than just fermentation, and no gas coming out can be caused by more than an absence of fermentation. If you want data, you need to measure the liquid with a hydrometer.