Compression marks occur from heavy items with small footprints being left on the carpet for a long period of time. Not only do the fibers of the carpet get matted down, but the padding underneath the carpet looses it's spring. Extra agitation and a little more time spent on these areas help, but even with all of the dirt and stains removed from the carpet you can still tell it's not new. To give your carpet a never occupied appearance, always put your heavy furniture on glides or carpet protectors.