I'll assume it's a Ford V8 (302 or 351?) and you're using the correct replacement pin. Most likely cause is a binding / stiff oil pump. The distibutor shaft has a hex socket that drives a 3/8" (approx) hex shaft that goes down to drive the oil pump gears.
Of course something in the distributor shaft or centrifugal advance could be jamming but that's much less likely and you probably already checked that, right?