Frigidaire dishwasher spray arms not alternating

I have a Frigidaire dishwasher that is about 4 years old now. It is the type that alternates spray arms. Before the problem, the pump would pause for a second, and when it restarted, the water would flow through the other arm. Now, the pump still pauses like it should (not a timer problem), but the water always comes out of the lower spray arm. I've checked every minute or so through an entire cycle, and only the bottom one works.
It is not leaking water, the pump is pushing water fine, and the middle arm rotates freely. The pump is just not getting water there. It seems to be a problem with the mechanism that changes which arm the water is directed to.
Any help appreciated.
Thanks in advance.

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