And remember, airlock activity is NOT the visible sign referred to. fterall that really is what an airlock is for. It's a valve to keep beer off your ceiling. If it's not bubbling, it doesn't really need to, it hasn't generate an excess amount of co2 to make it go blip. Or it is getting out elsewhere, like around the seal of the bung. But either way, as you can clearly see, airlock bubbling, though nice to watch is really irreverent to the brewing process. Some airlocks bubble, some don't. Some don't bubble for awhile, then start (but that doesn;t mean fermentation hadn't already started) some start and stop (and that doesn't mean that fermentation has ceased. Some bubble fast, some bubble slow....
It's just bubbles.....I have 9 different fermenters, and half of my beers I never notice airlock activity in them....and I've never not had a beer turn out just fine.