We spent a lot of time in the Delta
before we left the Bay Area.
As you know, most of the Delta has a muddy bottom with some pretty strong currents. The afternoon winds tend to blow hard in the summer.
Since I'll assume you tried letting out more scope
... you might want to consider a stern anchor
keeping your boat into the prevaling winds, which I believe are mostly from the west there during the summer.
You also might want to find other places to anchor with better holding bottoms. Head
down the San Juaquin River and check out Potato Slough, Three River Reach, Mildred Island and the 50 or so other anchorages
throught the Delta.
Another great resourse is the Stockton Sailing Club, the only Sail Boat Yacht Club in the Delta. Many of these guys and gals have been sailing the "ditches" for years and would be happy to give you some secret anchorages