One thing I've noticed is, that the lower the light intensity, the fluffier the buds. Likely, even if your main cola is rock-hard...the lower (secondary) buds are light and fluffy. (unless side-light is provided throughout flower)

Tried growing with 4' fluorescents a few years back, and all of the buds were very light and fluffy. I purchased a 400w HPS's and the same strain provides large, rock-hard buds that take forever to dry and cure. Same strain, (Swazi Skunk) same soil ,(Miracle Grow) and same ferts and micro's, same schedules.