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Could be that the unit is not getting enough water.
Most units fill to just below the heating element.
Let the unit fill then add a couple quarts of hot water if the unit now washes correctly odds are high that this is your problem.
The most likely culprit is the filter screen on the input side of the water valve.

Open it up and clean the filter.
Do not remove it as it is there to keep crud out of the valve, which could hold it open and cause a flood.
Note: Also a good idea to drain some water from your hot water tank crud accumulates at the bottom of the tank making it less efficient and contaminating your hot water lines.

Also could be crud/debris down in the pump area causing poor water circulation.
Also some of the wash arm holes may be blocked. Easiest to clean when wet using a pair of tweezers to pull the crud out.