I wouldn't pitch more yeast right now. I'd follow Wyrmwood's advice and just wait and see what happens. Or if you really want to know now, take a gravity reading today and another tomorrow. If there's a difference, the beer is fermenting.
A loose seal on a fermenter lid can prevent your airlock from bubbling, as can a number of other factors. I've brewed many, many beers that came out great and for which I never saw or heard a single "bloop". As many on this board have noted in the past, the airlock is not an accurate indicator of fermentation - a gravity reading is the only sure way to know if the yeastie boys are during their job.