How are you defining activity? bubbles in the airlock? or krausen appearance? Staring transfixed at the churning mass of yeast through the side of your carboy?
If bubbles, it could be that it's a bit colder than other batches so things are going slower, or the seal around the bucket lid/carboy stopper might be compromised.
if krausen appearance, I suppose it's possible that less hop utilization would cause less foam formation during fermentation. As Denny says, if you are having issues with head retention try more hops. If you used the same amount of hops but used a bag instead of free range you might well notice a diminution of the hops flavor, aroma, and bitterness because the utilization won't be quite as good.