Vokera Combi Boiler?

I am a building contractor and a sub contractor fitted a Vokera combi boiler about 3 years ago after about 4 months it kept cutting out and had to keep resetting it and topping up the water. Vokera came out and said there was now fault with the boiler. As time goes on the boiler has got worse and will work for a couple of days then cut off again it is reset and then cuts off again. Our plumber has informed us that Vokera does not want to know as it is no out of warrenty. Has any one got any idea why this is happening.