If you look at all of the "my airlock's not bubbling" threads on here on any give day, you see that it is quite common. And it doesn't matter, and airlock is a vent to release excess co2, so is a leak, either way gas gets out and beer stays off the ceiling.
The yeast doesn't really care how their farts get evacuated. It doesn't alter fermentation in the least.
Some fermenters don't even have grommet holes for airlocks, you're just SUPPOSED to leave the lid loose. Or some folks just put plexiglass on top, or saran wrap even. You have to realize is co2 is coming, nothing is getting in. No fermenter is EVER airtight...if the were then we'd have ceilings full of beer. How you achieve that lack of airtightnes, (or actually co2 tightness) is irrelevant.
Wild bugs are not ninja acrobats....all they do is fall in. They can't negotiate a gap where a piece of plexiglass for example meets the top of a bucket. The only concern is really insect control and a brick on top of the loose lid will keep the fruit flies and other critters out.
Counting bubbles does not equate to anything usable in fermentation. It's not like "x bubbles/minute= y gravity points." It just means that co2 is being released....but it could also NOT be bubbling, and still fermenting away.
In fact you might find
this discussion on the superfluousness of airlocks something that will help you get a handle on this. It was started by a newer brewing who just grasped this concept.