Unequal water supply from the overhead tank in four storey building with 22 flats - Why this problem and what is the remedy?

I am living in a four storey building with 22 flats, nine on each side and 4 in the middle. The overhead water tank which caters to our water needs has two outlets at the same height. One for nine flats and the other for 13 flats.

The nine flats get water for more duration i.e. even after the other side stops getting water (after the 13 flats stopped getting water). The plumber says all parameters are okay. Then why the unequal supply and what is the remedy? The problem is more acute when the tank is only half full as only the side with 9 flats get water from the tank. Please throw some light on this problem so that we can correct it.

Thanks

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