Thank you for your question.
I am sorry you are having a problem with the dishwasher. Without a model number, I can only assume which dishwasher you have. The older units plug at the check valve for the drain. To check this area turn the power off to the unit and remove the lower access panel. The first image shows the drain hose connected to the check valve. I recommend that you
shut off the breaker
to the dishwasher and remove the bottom front access panel to access this end of the drain hose. Use a shallow pan to catch any residual water that will drain out of the dishwasher when you remove this hose. Remove the other end of the hose from the garbage disposal. Blow air through the drain hose to verify that it is clear. If you have an air gap installed, you will need to check the line between the check valve and the air gap and then the line between the air gap and the garbage disposal. The second image shows the check valve with the drain hose removed. Unscrew the check valve and remove it to check the flapper valve as shown in the third image. According to your details, you may have already conducted these checks in the proper manner. If not, I recommend that you recheck these issues. If you find no problems with the check valve and drain line, I recommend that you check the opening of the sump where the check valve screws in for a clog or restriction. If you need to access the sump area from inside the dishwasher, this link of a previous answer shows how to disassemble the pump:
How do I access the drain impeller on a Kenmore model 665.16759791 dishwasher that wont drain?
These tips may help you resolve this drain issue in the dishwasher. If you need more help, submit a model number, additional details and we will try to assist you further.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services