Pump Pressure low on Filter but great on Recirc. Why?

If the water is really dirty, a DE filter will behave like you describe. They can require almost continuous backwashing. When the grids are dirty, sometimes only a small area is open, and that area can clog pretty quickly since all of the water (and dirt) flows through that area.

Thanks for the help , finally my pool is blue and clear. I filled to above the skimmer and then vacuumed to waste until I could see the bottom. Then when all visible debris was gone I soaked my grids in automatic dishwashing soap and the scum just melted off. Now my pump is at it's normal range of 10 psi and thats where it has been for the last 4 hours without missing a beat, once again thank you for the help.

dj,
Great news. Thanks for letting everyone know how it turned out.