The oil going up to the top end not only passes by the bushing in the cam chest, but also is fed to and from the cam cover and through the cam cover gasket. ¹⁾
It is possible for the gasket to blow out and seal up the oil galley feed passages.
Also if silicon was used on the cover, it is possible for that to break loose and stop up the passages.
You can take the cover off and run a tap down all the threaded holes to clear out any obstructions.
Also check that the face of the cover and case are not marred by some big scratch or dent.
To get old silicone out of bolt holes, you can use an inflation needle for footballs/basketballs/soccerballs. ²⁾
Cut the tip off of it then screw it into the end of an air blowgun.
It's narrow enough that it doesn't block debris from getting out of the hole.

Oil path of 85 style cam cover 3)

The feed to the rockers is not a large diameter passage and not a lot of oil goes to the rocker gear. ⁴⁾
But if you pull the lines off with the engine running you should be getting a good steady flow out of them.