- If you're using a plastic bucket, make sure the seal on the top is tight. Gas can escape out of the tiniest spaces, so a loose fit will prevent the airlock from bubbling.
- If you've got a glass carboy, or other clear container, and you see a krausen forming, you're fine as well.
- Sometimes fermentation goes by quickly, and it is possible to miss the entire thing.
- Check the gravity to see if it has fermented.
- If it is none of these things, wait 3-4 days total before pitching addition yeast.