Powermax 155x Lack of hot water?

I have a powermax 155x with expansion tank in the loft, the supply of hot water is very minimal both in volume and temperature. The blender value has been replace about two months ago which seemed to help for a short time. Do i need to get the Heat exchange replaced or is there not enough water in the boiler causing it to shut off?

I have checked the tank in the loft and it is full.