why does the water level in my vaillant combi boiler keep dropping? ?

i keep filling up my water level in my vaillant boiler but the water level seems 2 keep dropping. i have been turning the valve at the bottom of the boiler n refilling it but it just doesnt seem to stay put. every 3/ 4 days it needs filling up again. is there a leak somewhere? and i have noticed that there is no steam coming out from the pipe sticking out of the boiler that leads outside could that be the problem! please help!!!!!!!!!