Hello, my name is XXXXX XXXXX I will help you with your dish washer. As you say the flap should close to the opening that is not in use when the dishwasher is running. To test this and prove that is what you need you can block the one that is not in use with tape for a short time to see if that improves the pressure to the top. If this proves to be the problem you can refer to the parts list link for item # XXXXX Let me know if the link does not work or if you have questions. Parts link Dave
I have removed the bottom spinner and cleaned out the little reservoir below the screen. Is there any other area that could be reducing water pressure?
Inside the pump there is a check valve flap that keeps water from going down the drain during wash. If it gets warped or damaged you will loose pressure to both arms but it will be more noticeable on the top. It is #5 in the parts link. Parts link Dave