Sounds a little crazy, but it's not unlike trout in different parts of a stream feeding selectively on different bugs. Except that with shad, the selective fish are moving up stream to you. So when you've worked your range over thoroughly, and particularly if you've seen shad pass by, it means a bunch of shad have refused your fly. Time to start changing flies. The trick is to work with different fly sizes in certain color ranges.
Size is pretty easy to gauge.
Color depends largely on water temperature. Once you establish the color and size range you feel applies, start changing your fly every ten casts. Let's say, for example, the river is 63 degrees, it's June 1st and the water is low. Put on a split shot. Go to smaller flies, with a split shot.