[Whyman]

Some time ago I did have a problem like this and it turned out to be the little valves at the bottom of each jet.

Iirc each of the valves is one way to stop the screenwash running back to the pump and also to prevent the concentrated screen wash from evaporating and the water that remains freezing. I assumed that the one way valves were sticking partly shut and only allowing reduced flow.

I disconnected the valves and had a good flow from the pump so replaced both of the valves and all was well.