vaillant ecotec pro28r, not enough hot water?

hello all, i have vaillant ecotec pro 28r combi boiler, problem is there is not enough hot water but heating is ok, a week before we changed its diverter valve cos it was leaking, but after install brand new diverter valve the problem solved a bit but not completely, according to the man who install new part when we open the hot water tap it goes on heating side so thats why there is not enough hot water even settings on full, dont know why cos after change new diverter valve this should not happen so he have no idea about it, pls help me to resolve this, many thanks.