I you've got a Krausen ring you've got fermentation. Airlocks don't always bubble. As long as you're close to a seal on your fermentation vessel you're fine. 3-4 of my kids don't seal well on my buckets, I get very little airlock activity on those, but they always ferment. If its in a carboy, as state above you can cover it, but seeing as its only getting a little secondary light you should be fine.