I don't think your pond has enough weed cover or finely divided structure for the grass shrimp to thrive. Life span of the shrimp is only one year thus the young ones need lots of cover until they grow to maturity and lay eggs for the next generation. A pond near me had lots of shrimp with small pond weed and water lilies. Owner added grass carp who ate all the small pond weed. Grass shrimp disappeared despite some water lilies still present, but the g.carp are now working on the water lilies by mostly just cutting them off.
