Most screenwashes contain inhibitors that are designed to prevent problems such as this, but the secret is to ensure the washer bottle remains as close as possible to full at all times. It's when the bottle is left half full air and washer fluid react, which can lead to stagnation and the formation of algae on the top of the washer bottle, which is often what gets stuck and clogs up the pump and the jets. One thing a lot of people swear by – and it worked for us, so there may well be something it it, is to drop a dishwasher tablet into the washer bottle to help clean everything up. Just make sure it has fully dissolved before operating the washers.