You might try using an arts-and-craft pipe cleaner in the hub. Since
there is no electrical/mechanical mechanism in the hub, it basically
has to be a clog. I would start by cleanning the hub as good as can
be, then crimp off the known-good lines. This would allow more water
pressure thru the bad lines. Hopefully this helps dislodge the
blockage.
Of course, this is easier said than done, if you're on a hot roof, or
up on a ladder. Good luck with it.
Robin