If we are talking about an FE engine, when you tore it down you will have gone through the oil galleries for the lifters with a brush, like a rifle barrel cleaning brush on a long rod.
Here is the rear of an FE block:
The two holes above the cam are the lifter galleries. They go straight through the block:
From the FRONT of the block they look like this:
Notice two important facts, the hole on the left of the cam is also a bolt hole for the cam's thrust plate and the one on the right side of the cam is recessed.
The top hole feeds oil to the camshaft...
... and then the crankshaft:
Typically, Chevy engine builders miss these plugs. You need all of them. After I install plugs, I clinch the edge of the hole with a chisel so the plug cannot push out under oil pressure.
Let's see pictures of your plugs. If one is missing, how would I know without seeing the pictures? - Dave