Check to see if the lid has a gasket on the inside, I think most brew buckets do. Plus, is the hole for the airlock sealed? Some have grommets, mine is just a hole with a rubber "cork". You might have not pushed on the lid hard enough to connect all the way around with the seal, I'm going to assume you've put the lid back on, if so, don't open it to check the seal. Every time you open, you allow potentially harmful oxygen to your brew. Sounds like a good brew so far, it'd be a shame to spoil it.