When I was threading the castle nut onto the top ball joint, after a couple of turns the ball joint itself spins. This meant I couldn't fully tighten the castle nut as it doesn't travel up the thread. I hope that makes sense.
Check the threads are clear on both the castle nut and the ball joint, i had this with one on mine where the nut locked up on some tiny bit of shit on the threads and spun the ball round.
Failing that, you can borrow my electric windy gun to buzz it on which may or may not help
Is it suppose to turn easily?