Looking at the only info in my Carver water heater user manual (Truma took over Carver) it could be air in the system after changing a bottle and may need several attempts to light it. It also states if leasure battery is below 10.5 volts. Very spars on info. 
You could try contacting Truma they may be able to help.
Technical Help 
Touring caravan and mobile home space heaters, water heaters, water filtration systems and Caravan Mover are no longer manufactured by Carver the product range having been acquired by Truma.
For specific information and spares relating to the individual products, such as space heaters, water heaters, water filtration systems and Caravan Mover please refer to Truma UK Ltd. The links below are designed for ease of use.
Contact Details:
Email	
technicalATtrumauk.com
salesATtrumauk.com
  ON EMAIL ADDRESS CHANGE THE   AT    TO NORMAL EMAIL SYMBOL AS THIS FORUM DOES NOT ACCEPT EMAIL ADDRESS IN NORMAL FORM.
Website	www.trumauk.com
 
Telephone	01283 586050
Fax	01283 586051
 
Address	Truma UK Ltd
Park Lane
Dove Valley Park
South Derbyshire
DE65 5BG
                
 Bob
                
 May 2012