If you can hear the pump motor working, the chances are the screen jets or filter inside the washer tank is blocked. From the tank, you'll see the rubber pipe that feeds the washers. About half way from the tank is a white plastic connector - gently prize one of the tubes off. First blow into the tube that leads to the screen washer. If that is blocked, try clearing it with a low pressure airline. If that's free, then its the tank filter. To clear this without removing the tank, try this - but make sure the engine is not hot or running. You'll need to cover all the electric connections near the tank with plastic, then lay an absorbent towel around the top of the tank to soak up water. Now, blow hard down the tube or use a low pressure electric air pump. This should blow the muck away from the filter so it floats to the neck of the tank - you need to blow all the floating water/muck out. Its also possible that the tank maybe lined with algae (looks like small pieces of grey or black skin) and if so, this should start floating to the surface. Keep topping up the tank and blowing the all muck out. I may be a good idea to contact a main dealer and ask what you should put in the tank, to stop any more algae from forming. If in any doubt, take your car to a main dealer.