Pressure rising even after backwashing, help?

[papaalan]

So we have an inground 17k gallon pool that uses a typical DE filter tank.
Noticed recently that the pressure was up around 20psi.

Backwashed several times but after each time, the pressure went back to expected 10psi and after a day or so creeped back up to around the 20psi area.
Pool maintenace company guy said add 3 or 4lbs more of Di Earth, although it doesn't make sense to me that it would make the pressure go down.
Would appreciate any insights here as am considering opening up the tank and taking out the filters to examine and go step by step after that.
Had lines checked for air leaks etc., all good.

Thanks In Advance for reading this and any feedback.

[waterbear]

When is the last time that you broke the filter down and cleaned the grids? They might be fouled. DE filtyers do need their grids cleaned periodically.