Clams not purging!!

Guys, I go to the beach, harvest fresh clams, come home and purge them with the 1/3 cup of salt per gallon of cold water method and let them sit for 3.5 hours and change the water twice and they are still full of sand! Am i doing something wrong/is there a better way to do this? they are a mix of purple varnish and cockle clams!