how can i fix my ideal c80ff combi?

my combi fires up on ch but wont always fire when dhw is required. i switch on the ch and it fires up then the water comes through the taps only warm though.
i have cleaned the 3 way diverter thru and checked all diaphrams for holes etc but they seem fine. the micro switches work fine even when swapped over (dhw/ch) and vice verser,i'm lost for ideas...help.?????