as champion cole said you have to find the sweet spot on a drop,and as Danny said when the fish are that deep usually they are not in feeding mode.
look for the place that will be a ambush spot,a rise on the drop rock pile log stump.a flat bottom that just drops sometimes will have fish but more times than not it won't hold good fish.watch bait activity on the surface.Use your depthfinder and spend more time looking and less time fishing for a day or two.remember too the deeper you are the harder it is to hit the spot every cast,use marker buoys they are your best friend.i have a little rock spot on a drop on nolin we had over a hundred fish in 2 days,a good limit both days,if you hit very far at all from it you wouldn't get bit but we could throw our jig on the spot 2 feet apart and doubled many times,lots of them were little 14" fish but fun.out of a 100 yard ledge you may have 5ft of productive fishing.
nota pro at all but just my experience.remember too bass are not the only fish in the lake some of what you mark may be other species just keep trying and if you mark fish come back every hour to see if they are biting.