Ian has already explained most of it, chances are the reservoir and the main (front screen and headlamps) washer pump is blocked with gunge from cheap screenwash. I hate spending good money on expensive screenwash but it's easier and cheaper in the long run than using cheap screeenwash.
Solution - remove the reservoir, pumps and filters from the reservoir. Strip it all down and put the non-electrical items in the dishwasher with the reservoir facing down.
Once it has washed and dried, rebuild it all using a smear of silicone grease on all the rubber seals/grommets to aid their sealing and revive them somewhat.
Refill with clean water and good screenwash. Disconnect the hoses at various points and operate the washers to ensure fluid is coming out at a healthy rate, working through the pipework all the way to the headlamp washers and the screen wash jets on the bonnet.
While you're at it you may want to upgrade your bonnet washer jets to the S/V40 type with the fan jets - straight swap, use a deep 11mm socket/box spanner on the bottom of the old jets and push up towards the bonnet to release the tangs that retain the jets, pull them out, disconnect then connect the new jets and refit.
Vast improvement in coverage and cleaning ability!
