you can sometimes clear a blocked jet by putting your mouth over it and blowing hard, the blockage may come back but at least it will tell you what problem is. There is also a non-return valve in the line to rear washer spray, (lift up tailgate and look at centre of top edge of rear aperture where tube comes through from car and into tailgate) this can corrode up and stick, or (as happened on my Jazz) can leak and allow water to drain back all the way to washer bottle, i just bought another in-line one from Halfords for about £2