So a couple of things to check here. First, as Kevinski noted, the 55 block will have the steel line behind the lifter cover. Confirm your line is actually in place.
Second, and this is a little more complicated, 55-57 used a fitting at the steel tube with a 1/16 hole that feed the rockers. For 58, which fed the rockers from a main port, the block was drilled with a 1/16 feed hole. The 55-58 motors used rocker arms where the feed groove inside each arm intersected with the dribble hole of the arm.
In 59, GM made the last oiling change to these motors. Direct feed from the main galley, but the feed restriction was increased to 11/32. This created a sort of pressurized system. So to make sure the oil dribbled, the 59 and up rocker arms had the groove off set from the dribble hole. So with a 55 block, you will want to confirm you are using rockers with the early style intersecting groove. If you have the offset groove rockers, that may be part of your issue.
Lots of confusion on the "drilled bolt." So lets say you have a 56 truck and in 1961 discover that you need a new motor. You head down to the chevy dealer and order a fitted block. These replacement blocks were of the later 59 and up style motor with cast motor mounts to accommodate the mounting arrangements for the various years. The problem was that you could not just bolt on the 56 head and rocker arm assembly on to the 59-62 style block without getting too much oil to the rockers. So those fitted blocks came with a cap and a head bolt that was drilled through the center. When using the 55-58 head and rockers, you installed the cap in the block and blocked off the 59-62 style feed, and installed the drilled head bolt which was an alternate path for the oil to the rockers and had the correct restriction for the early style rockers. If you were using the fitted block with the later 59 and up head and rockers, you discarded the plug and bolt.
