The check valve may be stuck open.  Its a common problem that can compromise the amount of water getting to the upper wash arm.  Sometimes, depending on the configuration of the drain hose, you can hear an "urping" sound during wash periods when the check valve is stuck open.

It's just a little rubber flap inside a plastic housing that the drain hose connects to.  To test it, you have to remove the housing.  Remember which end is which, and the direction water flows through it when the machine drains.  The flap is supposed to remain closed except when the machine is draining and the force of the water pushes it open.  Blow into the housing from both ends.  If you're able to blow through it from both sides, the valve is bad/stuck open.  You should only be able to blow through it from the end that connects to the pump.

There may be pictures of the part if you want to do a search.  GE makes a replacement that will work, but it's only the flapper.  It's their part #WD1X1486.