The rocker shafts, rocker arms and rocker spit holes are somewhat notorious for getting gunked up and sometimes clogged with sludgy dirty oil. The fact that the rear rockers are getting oil is a good sign that you are probably generally getting a sufficient supply of oil to them. So if it were me, I would likely pull the rocker assembly off and give it a good thorough cleaning with solvent, including each individual rocker and running a pipe cleaner with solvent through each spit hole, and making sure the oil groove inside the rocker is clear and clean. Scrub the shaft clean clean clean. Make sure you keep track of the rockers, they must go back in the same exact order. Also, do not mix and match the push rods. Make sure they either remain in place or if you are removing them, keep track of where they go.
Finally, look at the ball on each of the rocker adjusters. The lack of oil may have messed them up. They are supposed to be round. If they are oblong or pointy, you will need to source a set of adjusters. That can be a bit of a challange but they turn up on ebay every once in a while.
