Hi Andrea,
there is a switch that senses the water level inside the machine. this works with a polystrene float activating a microswitch which to tell the machine when the tank is full of water. there is probable a lot of fluff built up around the polystrene float, so it is unable to bob up and down easily.
i hope this helps
www.candmdomesticappliances.co.uk
paul, June 2008