How can I make my Hoover TC650 / TC 650 start?

I have had a Hoover TC650 condenser dryer for 3 years. Over the past few months it has developed a problem. Whenever I empty and return the water cartridge the unit won't start up. It seems to think the water cartridge is still full. I have found that I can get it to work again if I dry the input valve of the cartridge with a cotton bud. However, this is a pain and doesn't always work first time.

Does anyone know what causes this problem, and is there an easy permanent solution?

Itchy