Rocker oiling or lack of .

If the lifters have what most builders would consider the “correct” oiling hole orientation/placement, then there is no direct connection between the oil hole in the lifter and the oil hole in the block.

The oil hole in the lifter would be parallel to the axle(for a Mopar), and above the oil band.
The oil has to squeeze between the lifter and the lifter bore to get from the oil band into the feed hole for the pushrods.
That is how the oil is “metered” to the top end.

If the lifter to bore clearance is too tight, or the band is at the wrong height so it’s not accessing the oil hole in the block...... then you get poor oil delivery to the top end.

The first check is to pull a lifter out and see if oil is flowing freely from the oil gallery in the block.