Hop aroma and flavor will fade over time, and is most intense right out of the fermentor. This is especially true with dry hopped beers (love me a newly kegged IPA... Wow). Exposure to oxygen during kegging and/or bottling will lead to diminished hop aromas, so flushing the keg and bottles with CO2 will help if you are not already doing so. Perhaps you need more hops in the recipe? I've nearly doubled or tripled my dry hopping addition lately to get those lasting and intense aromas (still notice fading over time though, so fresh is better in terms of aroma).