What you experienced is pretty typical most of the year. There will almost always be fish holding on major drop offs and as a general rule the fish pushed up on structure will be more active and likely to bite a lure. You must keep in mind the fact that the good ole kitty cats like to hang out on those ledges as well. You may have a hard time getting them to bite a slab. (unless you�re on tawak haha). In the underwater world action brings more action and I�ve had success starting a drift up shallow, stimulating the fish, and slowly dragging the school off the ledge and out over the deeper fish with them joining the party as a result. It�s a killer little trick. Watch your electronics and look for those deeper fish to suspend and move up in the water column if they are active. If you are successful in making that happen the fish in thirty will be up and feeding at the same level as the fish on top of the structure. So in your case I would keep my lure at 20 feet even in 30 feet of water.

There are other ways to stimulate deep fish that are mentioned on here from time to time as well.