I would first check to see if oil is exiting overflow tube, on the affected head, with motor running. If it is not then like R Pope said, there is a fault in the area of the cam bearing, or there is a plug in the oil passage, most likely on the top of the block where it makes a dogleg before going into the head.
If it is a problem with lack of oil getting to the rocker shaft, then there is probably a galled shaft and grooved, worn rockers as well. Check the bores of the rockers.