7 cups of DE, as in seven 8-ounce measuring cups??? Way too little DE.

There should be a label on the filter saying how much DE to use. A 36 ft2 DE filter usually should take about 3.6 pounds of DE. An empty 'pound' coffee can will hold about 1/2 pound of DE powder, so you'd need 7 cans full of DE to get your 3.5 pounds of DE (close enough to 3.6 pounds).

As to the pressure, maybe your pressure gauge is NG and needs to be replaced. Or you don't have to valving set properly and you are starving the pump of water and hence have a very low pressure. Or you have a suction-side air leak and hence have low pressure. It could be many things, or maybe that is just the pressure that your clean filter runs at.