Usually carpets that are older than 15 years are a lot harder to stretch. The backings of the carpet were made differently making them more rigid and the backings of carpet gets harder and less flexible with age.
Fortunately the power carpet stretcher is able to take out ripples/wrinkles in even the stiffest carpet, this was the case today as the carpet was just over 20 years old and it resisted stretching. It resisted stretching to much it was like stretching concrete, or like stretching a sheet of plywood. There is no way a carpet knee kicker would have even touched stretching this carpet even for the man of steel. A proper stretch on carpet should last it’s lifetime.