Question for the experts?

[Big Blue 72]

I have a problem I've never seen before and I work on pools during the season.
I installed a new hayward 100 lb sand filter system and new 1 hp pump, for a family whos old Hayward sand filter developed a crack at the seam near the collar. The system runs great ,pressure at the return and suction sides . When you switch to "waste" it still has good pressure but when you try to back wash ,the pressure reduces by about half.
This baffles me . I installed the whole unit being careful about the sand filter assembly, and added sand with the down tube covered.
I've inspected the sand itself, clean and granular filter sand.
Any idea why running it on back wash lowers the pressure?

 

[JamesW]

Do you have a picture?

 

[Oly]

Less head pressure during the backwash due to plumbing difference is my guess . Where is the backwash water plumbed to?
To be clear I am no expert but I have learned a thing or two the hard way.

 

[JamesW]

I suspect that the multiport is, or was, plumbed incorrectly.

The laterals might be clogged from the inside.