Undissolved detergent can be caused by your dishwasher’s detergent chamber being blocked from being able to completely open by an obstruction like a large or overhanging utensil or dish. Before adding detergent, check to make sure the chamber isn’t wet, as this can also cause powdered detergent to not dissolve completely and stick to the dispenser.
Consider trying Cascade Platinum Pacs™. They are designed to dissolve quickly right from the start of the wash cycle.