Since you did not include a model numer the following are just general things to check

1. Remove the sump cover (center bottom of tub) and check that it is clear of debris and that there is nothing wrapped around the chopper. Be careful as there is often broken glass in this area. Also when in there rotate the chopper and look at the armature on the pump motor to ensure that it is not slipping and is connected.

2. Check that the pump runs when it should.

3. Check that it is filling OK. Most units fill to just under the heating element. If it seems low, open the door just after the fill and add a couple quarts of hot water. Be careful to not overfill it. Then run it to see if the arms rotate properly.
If yes remove the fill valve from the unit. There is a screen filter on the input side of the valve. Check/clean this. Do not permanently remove it as it is there to prevent crud from getting into the valve, holding it open and causing a flood.
If it still does not fill correctly, replace the valve.

4. Some units have a small flapper valve in the pump throat. Inside the tub just under the lower spray arm. If this has rotted out of fallen out then the water shoots out of this hole and can greatly reduce the water pressure to the arms.

5. Make sure the flapper valve returns properly to the wash position (up)
There is a wash/drain solenoid on the left hand side of the pump assembly.

6. Check that the holes in the spray arms are clear. I find the easies way to clean them is to run the unit for a minute then stop it and use tweezers to pull the debris out of the holes. You will have to do this several times.