Probably too much water rather than too little. Planting in clay rather than above it often leads to water logged soil which in turn leads to the roots rotting away. Adding top soil does nothing to make the drainage better. The only real solution for clay is too ceate a raised bed on top of it. It may ,unfortunately, be too late to save this particular plant given the symptoms you describe. You can try digging it up and potting it in a suitable medium to see if it revives.