He probably means his airlock is not bubbling, which really doesn't matter. Just like mentioned in here;
https://www.homebrewtalk.com/f11/carboy-caps-not-air-tight-what-131116/?highlight=airtight
Neither are some bucket lids, or stoppers in carboys, or the gasket where the airlock goes into...that's why many airlocks don't bubble...But the good news is things don't need to be air tight. In fact that's what an airlock is for to void built up pressure so you don't paint your ceiling with your beer.
See, the thing is fermentation makes co2...which pushes outward, and if co2 is getting out, then nothing else is getting in...including air.
The only time we really need something airtight (actually co2 tight) is in the bottle or the keg....because that is how our beer get carbonated, the co2 cannot escape so it permeates the beer instead.
If you look around you will see that a lot of brewers don't use airclocks, or lids, or stoppers at all, some use tinfoil instead or even place a piece of plexiglass on top of their buckets instead of the airtight lid...any co2 that needs to get out, will....and the positive pressure from the co2, keeps anything else from getting in.
Airlock bubbling, lack of airlock bubbling, stopped airlock bubbling, fast airlock bubbling, slow airlock bubbling, heavy metal airlock bubbling, or disco airlock bubbling really is not an indicator of what is happening to your beer. It is NOT a fermentation gauge, it is a valve to release excess pressure, excess CO2...NOT AN ACCURATE INSTRUMENT....
Your Hydrometer is the only gauge of fermentaion you should use.
It's a valve to release co2...and like I said, if co2 is getting out,
nothing bad is coming in.
