usally if its not seating the pump shaft is not lined up. got to move the pump shaft a little so it will lineup. just got to play with it a few times. never goes in the first time. make sure the gear and oil pump drive have a little oil on them also a little oil on the top area where it sticks thru the block.
Most of the time this can be fixed by rocking the engine back and fourth a little. But dont turn it all the way over just give it a little bump to get it finally lined up. Just wiggle the crank a little dont turn it at all if the dizy gear is not lined up.
Make sure the oil pump is lines up before you bolt it down. do not try to force it or hit the starter motor to get it seated.
Does your car have an oil pressure guage? Also make sure the wire is connected for the oil pressure sender. You want to make sure at least the warning light will come on if the car has no oil pressure.