Sand Filter Help !

Hello all, my Mom has a IG pool with a sand filter (triton T100 I believe it is ). Last week her Hayward cleaner stopped moving as well as it use to so she took it in and thay replace the flappers but stated to her that it showed signes of being used with too much suction (not sure if related). She reinstalled the cleaner with no improvement, notice her pressure was high on her filter so she back washed. The pressure did not drop at all but instead remained at about 22psi. I stopped by last night and she told me of teh problem so I tried back washing as again the pressure did not come down at all. It apears to me that he filter has a clog at some point. Where is the best place to start looking?

I read somewhere that there is a chemical cleaning process that can be completed prior to rip it apart and changing the sand.

Filter appears to be 10+ years old, dhe has owned the house for 5 and it has never been cleaned.

Next steps ?

Should have a bypass or recirculate position on the valve. See if the pressure drops on that setting. If not then the blockage is somewhere else. If it does drop, see how much and this will give an idea of how restricted the filter is. She didn't dump anything in the system that would goop it up did she? There isn;t much that can go wrong with a sand filter. See what happens and we'll work it from there.
Al