It is also really important that the wort is 65-75º, if you take Yeast that is room temp, dry, or out of the refrigerator and toss them into your wart that is 75-90º they go in to shock.
One thing also about aerating, the more O2 there is in the system the longer the yeast will convert sugar completely to CO2 and H2O, they need to be in an Anaerobic state due to the lack of O2 to produce EtOH (ethanol) and CO2. So if you oxygenate the system the yeast will be more active but produce less EtOH from the available sugar, but more bubbles.
I find it is best to have as little O2 in the system before pitching the yeast, but to try get the yeast in a linear growth rate at the time of pitching...basically activating the yeast 3-4hr before pitching (with dry Yeast re-Hydrate for 10-15min before adding to sugar solution), if you do this the yeast will be dividing at a rate of one division per 24hr and do this for 2-4 days until the system is saturated.
Some times the yeast are not as healthy from drying or whatever, they just take a while to get to the numbers necessary for a robust fermentation, but they will get there if you give them a chance.
P.S. you need 50million yeast cells/ 1ml of wort...