Funny thing, I just had a similiar service call yesterday. I found that the discharge hose from the dishwasher to the kitchen drainline had shifted somehow and now was pinched off, leaving some water in the bottom of the dishwasher. Check the discharge hose for any pinching or their could also be a blockage inside the line itself. If you can disconnect the discharge from the plumbing and aim it into a pail. Then run the dishwasher so that it fills and then discharges, you can see by the flow into the pail if there is an obstruction inside the hose itself.. I’ve never heard of installing a checkvalve in a discharge line. The pump itself may be failing, or not working propery as well. Good luck.