I have the same push pull valve as you. When I backflush I repeat the following procedure 4 or 5 times.

1. Turn off pump
2. lower valve handle
3. Turn pump on to remove DE, let run 1 minute
4. turn off pump
5. raise valve handle
6. Turn pump back on for 20 seconds


repeat steps 1 through 6 at least 4 or 5 times. This gets most of the DE out. try that next time.