Lavender's fantastic piney/floral fragrance is one of the reasons why it is so popular. After it has been sitting around for a while undisturbed, however, the fragrance fades. That doesn't mean it is time to get a new bundle - if you crush a single bud between your fingers, you'll be hit by a burst of scent - it's still in there, but needs a little coaxing to come back out.
Enter the hair dryer. Put it on its hottest setting and blow away. The oils will migrate out, filling the room with fragrance again.