The common reasons why a dishwasher is not draining include:

• A blocked or clogged filter due to food debris, grease, and dirt buildup that prevents proper water flow. Cleaning the filter regularly often solves the problem.
• A clogged, kinked, or crushed drain hose that restricts water flow out of the dishwasher. Straightening out kinks or removing blockages in the hose can fix drainage issues.
• Foreign objects or debris stuck in the drain pump, which can jam the pump and stop it from working efficiently.
• Mechanical or electrical problems with components such as the drain pump, drain motor, or control board that may require professional repair.
• Using the wrong type of detergent (like regular dish soap) that causes residue buildup and clogs.

Checking and cleaning the filter and drain hose are the easiest first steps. If those aren't the cause, inspecting the drain pump or seeking professional help is advisable. Additionally, improper dishwasher installation with issues like incorrect drain hose positioning or plumbing connections can also cause drainage problems. In summary, the most frequent causes are a clogged filter, blocked/kinked drain hose, or pump issues, and these should be checked and cleaned to restore proper drainage.