I seem to be answering this one a lot today...
Well actually bubbling in the airlock is not a true indicator of fermentation, the hydrometer is...So in a sense the question is moot....The airlock is outgassing oxygen initially and more importantly co2 (which is of course a byproduct of fermentation), but there are a lot of variables that come into play in terms of the airlock bubbling...You could have for example and bad seal between the grommet (or stopper) and the airlock, or the lid on the bucket isn't fully tight and gas is getting out elsewhere besides the airlock, or the stopper and mouth of the carboy and that would appear slow, while fermentaion is actually occuring rapidly... Or the little bubbler in the airlock could be sitting a little crooked, or become weighted down with tiny co2 bubbles and need to build up a good head of gas before blurping again.
Even having the airlock leaning slightly askew affects it. half my beers have almost no airlock activity.
So as you can see airlock activity varies, and should not be used a a sign of speed or lack of fermentation.
Get outta the habit of thinking that airlock activity means anything..it is a vent and nothing more to keep the lid/stopper from blowing off your fermenter and painting your ceiling....it is a cheap piece of plastic and not a precision instrument...so don't treat it like that...
You know what is the TRUE indicator of fermentation, don't you, the only accurate measure?
Before you contemplate repitching or anything make sure there's a problem...read this before you do or contemplate anything.