A customer who we laid sod for a couple of years ago won't stop complaining that his lawn is "lumpy". The sod took very well, but when you run a mower over it you can feel there is divets all over it. Its almost as though someone walked through it when the topsoil was soaked and left footprints. I've tried aerating, rolling repeatedly, spot-topdressing, and power raking. Any other suggestions?