I'm guessing that the pump is OK for the filter.
The use of DE to test a filter requires more DE than you use to improve filtering. Several of us mods use DE to improve our sand filter's performance and have for years.
1) Back wash.
2) Get your base pressure reading.
3) Add about 1/4 cup of DE to you skimmer while the pump is running.
4) Wait 20-30 minutes then check the pressure reading
5) The goal is to figure out how much DE it takes to raise the pressure by 1 pound.
6) If the pressure has not increased by 1 pound, add another 1/4 cup of DE and wait 20-30minutes again.
7) Repeat until the pressure has increased by 1 pound.
8) If you add too much and return flow drops, or the pressure goes up more than 1 pound, simply backwash again and repeat.
9) Note how much DE it takes to get to the 1 pound increase.
10) Every time you backwash you will need to add that amount of DE to the skimmer.

You should notice that the water is clearer, with an almost polished look.