Diagnosis
The top spray arm isn't getting enough water pressure. This is usually because the upper arm is clogged with hard water scale or debris, or the supply tube to it is obstructed.
How to Fix It
Remove the top rack. The spray arm up there snaps off its mount. Pull it straight down to remove it. Check all the holes for clogs—use a toothpick or a pipe cleaner. Also, look into the water inlet hole on the arm's mount and clear any debris there. Soak the arm in vinegar if you see scale.
If That Didn't Work
The water inlet valve that feeds the upper spray assembly might be failing. It's less common than a clog, but if water pressure is low everywhere, that valve (part #00482610) could be the culprit.
Heads Up
Don't force the spray arm off—it's plastic and the mount can break. A firm, straight pull is all it needs.