An airlock is just a vent, a valve to release excess co2. It's not a fermentation gauge. Sometimes they don't bubble. It doesn't mean anything. Fermentation is slowing as more and more sugar has been consumed, and consequently the yeast are farting less, so there's less EXCESS co2 needing to be venting out. Nothing more.
In a few days you can take a gravity reading if you want. But remember an airlock is not a gauge.