There aren't that many things that can cause low oil pressure. As gmck suggests, begin by assuring that your testing method is good.
Not specific to this particular engine, but rather in general, internal parts clearances on rotating assemblies that are too great will cause low oil pressure. A problem with how the pump is driven, sheared pin, key, damaged gear drives will cause low oil pressure. A missing oil gallery plug or misaligned or improperly installed gasket will cause low oil pressure, as will damage (crack/hole) to a block, case, housing missed during inspection. Sometimes a part will fit two ways but only one way is proper becasue oil holes have to line up. An example in an OHV car/truck engine is cam bearings; if the oil holes don't line up with those in the block no oil get's past the camshaft bearing. Some head gaskets will fit two ways but only one way lines up with oil galleries. If you are testing downstream from a situation like that you will get low pressure readings.
I don't know your experience level with the particular engine in question, and it's not my intention to cast an dispersions. I have rebuilt several engines, but none of this type. If I had this problem with an engine I hadn't previously dealt with I would be questioning myself about whether I missed something during reassembly. If this is your first, sometimes it's easy to miss the small things and often service manuals aren't clearly written. Unfortunately, if you can't pinpoint and fix the problem from the outside you may have to disassemble and inspect your previous work to find the problem. If that turns out to be the case don't feel too bad, you won't be the first.
