Go to the website look for 'Ask Thetford' drop them a note asking what can cause that using there online form.
Below is their pdf repairs just find model of your toilet which you will need when you send them a message. If unsure look up the various ones on their site and select which is like yours.
For those is Australia www.thetford.com.au the access for pdf repair downloads may differ as I haven’t check if it changed like the Europe website.
www.thetford-europe.com
On home page click on Products
Click on read more (Toilets)
Click on read more (cassette toilets)
Click on read more (C200)
Click on Service
Click on pdf Repair instructions- sanitation
Save file to computer then on disc for reference when in caravan obviously with DVD/CD player.
Bob
January 2012
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.