Dishwasher detergent powder; New

New, unopened 64 oz. (4 LB.) tub/container of Wegman's brand Plant Powered dishwasher detergent powder. We used up one tub but don't want to go through another. The powder doesn't dissolve easily. We mix it with hot water before adding it to the dishwasher but have to use half a scoop. I tried boiling water with the recommended amount (full scoop) and it still leaves a grainy residue on the door. Maybe it will work with your dishwasher.