What kind of fermenter are you using? Carboy? Bucket? Other?
If a carboy, are you sure you have the stopper pushed into the neck all the way to form a tight seal?
If a bucket, the lid on the bucket may not seal well and there is likely little you can do about it.
The common disclaimer is.. bubbling is NOT a sign of fermentation. I'm assuming (and that's dangerous) that you are not using a carboy as you'd SEE activity going on.
When did you pitch your yeast? Yesterday, Today, Three days ago???
What was the temperature of the wort when you tossed in/pitched your yeast? Also, if you kept the yeast packet.. did it have an expiration date on it?
What temperature are you fermenting at? It could well be a higher temperature than you think because yeast metabolism creates heat on it's own.
Also, did you take an Original Gravity with a hydrometer? The hydrometer really is your friend. If you know where you started.. and three/five days from the start you should see a significant drop in specific gravity... but, if you read the "sticky" at the top of this beginner forum, you will see that.. yes, it can take 24-72 hours for fermentation to start. Never been more than about 12 hour for me.. but, I'd keep that in mind.
Anyhow, you should really post more information about:
What are you brewing.
What was the recipe
What was the yeast
What is the temperature in the fermenter?
probably other Etc's
Let us know..... We are here to help