I have a S21OT sand filter that is running off the pressure guage and flowing sand back into the pool. The laterals are fine, I changed the sand, backwashed for about two minutes, and rinsed, The pressure is still the same. Any suggestions ?

Is there any chance that when you changed you sand you happened to grab like playbox sand as opposed to the more coarsely ground pool grade sand. Outside of that I would check the standpipe and lateral assembly again because those are pretty much the only two causes. Also when you put the new sand in, might you forgotten to fill the filter tank about halfway with water or maybe even gotten some sand down the center pipe? Is there any chance that when you changed you sand you happened to grab like playbox sand as opposed to the more coarsely ground pool grade sand. Outside of that I would check the standpipe and lateral assembly again because those are pretty much the only two causes. Also when you put the new sand in, might you forgotten to fill the filter tank about halfway with water or maybe even gotten some sand down the center pipe?