What type of clam do you have?
It's perfectly normal for clams to "hop" around a bit. But they do tend to skip more when they are distressed. And for rock borers like crocea and maxima they don't move as much once they are anchored. But my 10" derasa and squamosal are sitting freely on the sand bed, and they all move a bit here and there every now and then, no more than 1/4" each time though.