saunier duval isomax c/h and hot water working but not hot?

Saunier duval isomax, when i run the hot water it works fine for 5 minuits due to it having a storage tank inside when this is used up it starts to run cold when i look at the flame it is verry low its just a level sheet of flame, when i put it just to heating the flame is the same and the heating takes a log time to get hot. I think that over the day when we dont use the hot water the boiler has time very slowly to heat up the small clinder....
Any help thaks