Could either be down to a blocked jet or pipe but there is also the possibility of dirt or gunge inside the pump - one pump serves both front and rear.
You could try flushing the reservoir with hot water (hand hot) as sometimes that can help the gunge dissolve
ive sorted it this after noon!
i opened the boot and removed the pipe that comes strate from the washer pump in the front with the boot open there is a little rubber gromet where the pipe enters
the inside of the tailgate housing and where the 2 meet there is a conection piece and that was full to the gunels with gunk !
cleaned it out with a pin and hey presto the back window now getting lots of screen wash!