There are a number if possibilities.
First, there could be to much clearance between the rockers and shaft due to wear. This would allow oil to leak out the sides of the rockers, preventing it from reaching the vent hole.
Since the engine sits at a downward angle #1 rocker is the highest and the first to starve if oil is bled off from it or the other rockers.
Second, since you said your overflow tube is pinched off we can assume you have a pre 1959 engine. Up to 1958 the groove inside each rocker is centered. If a 1959-62 rocker was used, which have the groove moved 1/4" offset, it will be hard for the oil to reach the vent hole.
Third. It is very common for the rocker oil restrictor orifice to become clogged. It has a 1/16" hole in it. It depends on the year as to where this restrictor is, up to 1957 it is the fitting at the back of the engine that the tube screws into. 1958 it is under the 1/8" pipe plug in the center of the engine. BTW these are both under the side cover.
There are others but I will stop there.
