I assume you have checked that the water is not leaking away somewhere?
If not, it could be an 'air-lock' where there is trapped air in the heater that prevents the water from filling it. If this is the case, you will need to 'bleed' the air out of the heater. Unless there is a bleed valve (like on the top of a house radiator) you could loosen one of the top hoses on the heater to let the air escape while someone fill the water reservoir. The hoses are difficult to get at but its worth a try. Another possible way is forcing the water into the reservoir under pressure to hopefully 'blow' the airlock out. You could try a hose. It's messy but it could work rather than having to get at the heater hose.
Best of luck. I hope this helps.
Barry, November 2010
