There are several reasons why airlock activity isn't reliable. One is simply that the fermentation happens, but maybe not vigorously enough (or only for a few hours while you were away) for you to notice bubbling. Also, sometimes lids and bungs don't seal competely airtight and you might have an airleak. Even though it might appear to be sealed, maybe the bung slid up a little bit. And sometimes the fermentation is finishing up and the sg is dropping but not producing vast enough amounts of co2 to cause airlock bubbling. Sometimes people leave quite a bit of headspace, and it takes alot of co2 to purge all that headspace and bubble.
Conversely, I've seen airlocks bubble pretty steadily. When I went to bottle, I checked the sg and it only dropped from like 1.060 to 1.030! Definitely not done- but stuck!
Steady hydrometer readings that are in the expected fg are the only way to know if fermentation is finished.