What causes pool pump pressure to rise rapidly even after backwashing to the point of clogging within six hours?

Mudballs in your filter, a blockage in the pressure side Plumbing. or a loose gate valve that closes? most likely mudballs in your filter. there are many different variables that can lead to mudballs in your filter. bascially though your sand turns in to a liquid concrete, and the water only passes through tunnels which the water have forced through. when you back wash, you are increasing the flow through the filter and the paths are cleared rapidly. over time, the disruption could cause a collapse again of those tunnels.. all in all, change your sand in your filter. or have a professional do it. I might be jumping the gun, but if it is a cartridge filter you have, they cannot be backwashed... the valving on those filters can be changed to "waste" to lower to pool water by , bypassing the filter completely. you have to remove those filters and chemically clean them everyso often. but in this case if it was a cartridge filter, i would replace the cartridge, they get to a point where even the chemicals can't clean them.