badhabit

Recipes 

It doesnt take much of a leak to have the air lock not move. In addition to the great info already given I would note that if your lid is not air tight you should be able to smell the wort fermenting.

MSKBeerfan

Recipes 

Thanks to all who have started and contributed to this thread.
I brewed an American Amber Ale on Saturday with a WLP002 yeast and did not have time to do a starter so I just pitched after cooling.
I was concerned that fermentation did not start as I have no activity in the air lock but much to my suprise when I opened the lid I have a great Krausen on top happily bubbling away... Not sure where the C0 is escaping to but the yeasts are getting there fill of sugar!