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?
The 'O' ring that stops water getting into the mechanism (ie the blind threaded hole in the part with the washer on it) wears, water gets into the mechanism, and you can't compress water. You'll find if you take the valve out and take it apart, some water will always drip out. It's yet another example of stupid design, solving the non-existent "problem" of old taps where the handle moved up and down with the washer. It goes along with push button operated loos which leak and waste clean water, as one of the most ridiculous ideas anyone has yet thought of.
It can also be caused when the washer wears / compresses with time. This enables the left hand threaded "slug" to move too far down, and sometimes come out of its thread. Solution in this case is a new washer of the original thickness.