A Sublime Master of Appliantology provides some good tips in the repair forum on troubleshooting this problem.

*Water level on this unit should be up to the bottom of the heating element (using the element as a face of a clock, use the 9 & 3 positions) when the water shuts off if filling correctly. When the unit is done filling and you get the pause prior to the motor kicking on, look at the float switch on the left hand side behind the kick plate. The float is down to fill and will rise allowing the plunger of the switch to extend and stop the fill. If you are not getting a pause between fill and motor operation, you might not be filling with enough water. If the float is still down, add until it raises and breaks the contacts. Shut the door and start the unit, do the arms now move? If so, could be clogged screens in the valve

* Filter in the sump. Remove the lower arm, remove the torx screws holding the top part of the pump housing on. This will expose a micro-mesh filter that could be clogged to the point you are starving the pump of water. This will allow water to come out the arms but not enough pressure to move them. If they are clogged, inspect the back wash arm, that is the 4 armed spinner in the middle of the filter. The arms have verticle slots in the end of them that spray water out to clean the filter. If they have become clogged, clean them and the filter.

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