If you live in a hard-water area it may well be limescale. If you poke a fine needle into the jet it willl just push the obstruction back into the pipe so it can block it up again next time you squirt...
Half a cup of Viakal in the water tank - leave it for an hour and have a squirt - it may be dodgy for a day or two until the Viakal has had a chance to work its way through the pipes.
Well it worked for me...
Peccavi
July 2009