This article is from the Beer and Other Brewing FAQ, by Kurt Swanson email@example.com with numerous contributions by others.
Some yeasts take longer to start than others. Make sure your fermentation
temperature is in the right range (lower temps slow yeast activity). Also,
high temperatures are bad for yeast. Besides problems of mutation, yeast
may be killed if pitched before the wort has sufficiently cooled. You might
try aerating the wort by sloshing it around in the fermenter. Lastly, the
pitching rate affects startup time. If you pitch too little yeast, not only
will the lag time be greater, but you also risk infection. Many people
either use 2 packets of dry yeast (Whitbread excepted), or make a starter
culture from one packet, or from liquid yeast.