Start with the filter
Most modern dishwashers have a removable filter in the tub floor that almost nobody cleans. A matted filter recirculates dirty water over every load. Rinse it under the tap and the difference is often immediate.
Check the spray arms
Spin each arm by hand and look for cracked arms and holes plugged with mineral scale or seeds. In Cheatham County, hard-water crust on the arm jets is the single most common cause of a top rack that stays dirty.
Water temperature matters
Run the kitchen tap until the water is hot before starting the dishwasher. A machine that fills with cold water washes cold for most of the cycle, and detergent never fully activates.
Loading is a skill, not a chore
Bowls nested tight, a cookie sheet blocking the spray tower, and silverware spooning together all defeat the wash. Leave gaps; the water has to reach the dirt to remove it.
When it is the machine
A circulation pump losing pressure, a failed inlet valve filling low, or a heater that no longer sanitizes all need parts and testing. If the filter, arms, and loading check out, book the diagnosis.