As suggested in the Projects section, it is likely to be a split diaphragm in the syphon. This is usually a piece of heavy duty polythene sheet. When mine failed, I managed to make a new one out of part of a strong plastic sack that had contained cat litter.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!