Most of the taps in my house are of the type with a captive nut with LH thread that moves up/down when the head is turned, carrying the washer. They all eventually fail as progressively less water flows until it's impossible to get any. When taken apart there's no apparent problem except that the captive nut seems to have progressed downwards (as if it has slipped a whole turn). There doesn't appear to be enough room inside the tap body for that actually to happen. Can anyone explain what goes wrong, please?