I once replaced a cylinder head gasket 3 times after having it pressure tested. I eventually found that there was a hole in one of the galleries in tha water jacket that was allowing water to leak from the gallery into one of the intake ports behind the valve (was not visible, even with the valve removed). The only way that I could find it was to remove all the valves and fill the water jacket with kerosine, then look for the leak.
This is a worst-case scenario and usually requires replacement of the head. Start with a compression test to see which cylinder is affected.