It's not clear from what I've read in this thread so far, but I thought I'd better mention that oil filters are one-way valves. Something I did not know.  Make sure you haven't reversed in- and outputs to the filter.

Finn

Ready to try a first start of the engine if oil pressure could be counted on to come up.

Perplexed as to why I get oil pressure sometimes and not every time I spin the engine.

Wendell (with oil all over the place)