I agree with above posts. I have had problems with either aeration or not a big enough pitch. If fermentation has not yet begun, or is within 12 hours of pitching, the yeast have not yet really started attenuating, just have started building more cell walls so your brew can ferment comfortably.
I rock my fermenter back and forth to rile our little friends up, and the 02 left in the head space should be enough to kick off the process.
I'm new to making beer, and got so wrapped up in the new processes that I neglected to properly aerate a batch ... as a result, nothing much was happening. I used my wine whip (drill mounted stir stick) to beat it up a bit, and it's taken off quite nicely now.