In message , Harry Bloomfield
writes

Sometimes there is a sort of jelly like substance which grows in pipes,
blocking jets. Easy way to clear them is with compressed are, blown in
from jet end of pipe, towards the bottle, making sure there isn't a one
way valve in the system.

The problem, now cleared, was not the jets. No idea whether they're all
the same, but on this car, the water comes through a hole in the wiper
spindle, to be met with a cap which turns the water through 90 degrees,
and directs it through two jets. The blockage was within the cap,
before the jets, and whatever it was, was solid. Softened by running
through the dishwasher, then soaking for hours in a cup of hot water
with washing powder. Much squirting of contact cleaner (which may or
may not have helped), poking with a tiny screwdriver and old guitar
strings and blasts from a compressed air aerosol. Now working
perfectly.

--
Graeme