1.5 hp pump... solar heating or no?
With a 48 Sq.Ft. DE filter, that is a large residential pool filter and should filter a pool with ease.
You are describing a similar symptom of one of my clients that also has a ProGrid DE filter. As I describe my clients pool, see if this sounds similar; Every year I go round and round with my client to cut his tree back away from the pool. He has a tree planted near his pool that drips this awful sticky sap. This sap drips into the pool and drips on his pool deck leaving the deck under the tree "sticky". This sap makes its way into the filter and clogs it up, causing his returns lines to sort of ooze out water rather than jet water out. His pool doesn't filter and turns cloudy and begins to develop yellow algae on the sides of the walls. If I go and backwash the filter, I can get some of the sap to wash out with the DE, but the problem is this, his filter pressure starts out at 15 lbs, then begins to increase to nearly 30 pounds of pressure. The problem is inside the filter, tree sap has clogged the filter grids. At that point, I have to completely disassemble the filter grids and by disassemble, I have to unscrew the main bolt that holds them all together, take out the grids one by one, lay them out on the lawn and individually clean each filter grid with cleaner spray and high pressure hose nozzle. IF I just clean the grids still assembled, I only get partial cleaning results and the problem with pressure and lack of filtration starts all over again, as though I never cleaned it. Once put back together again, with new DE, his filter pressure is normal again around 17 pounds. My client gets the gardeners to cut the tree back so the branches are no longer over the pool and his pool is crystal clear with good excellent water flow, at least, until that damn tree grows back again in the Springtime.
Have you performed a thorough cleaning like this?