Too little water is coming out of the windscreen washer jets. What can cause this?

You answered correctly. If too little water comes out of the windscreen washer jets, it can be due to the jets being partly blocked or a defective washer pump. Blocked jets restrict water flow, while a faulty pump cannot deliver enough pressure. Too much water in the reservoir would not cause this problem. In the future, remember that poor water output is often caused by blockages or technical defects in the washer system, not by the amount of fluid.