Is it in a bucket? If so you probably have a leak in the lid seal and the airlock might never bubble. This is nothing to worry about. In about 10 to 3 weeks take gravity reading a day apart and if they are the same your fermentation is done and you can bottle.
You can also lift the lid and look for krausen, a layer of foam on top of the beer. If there is one your beer is fermenting.
Also what is the temperature of the wort. I like to ferment US-05 in the low to mid sixties. I have one going right now and it started in about 18 hours.