Twenty seven hours isn't out of the question. I oxygenate with pure O2 and sometimes it takes my yeast a day or so to start so I wouldn't jump to the conclusion that aeration is a problem, or even that you have one. Do you see krauesen, the foamy yeast on top of the beer? If so, your beer is fermenting and your lack of airlock activity is likely a leaky stopper or bucket lid. If you take a gravity reading and it's less than your OG, your beer is fermenting. But I'd caution against reoxygenating: Introducing oxygen after a day or so into fermentation just shortens its shelf life and increases the chances of staling.