One of the four posts doesn't reach through the base far enough for the threads to grab the nuts that are installed on the bottom. At least, it won't reach through far enough with one of the knurled nuts installed. If I take it off, of course it goes all the way through. What can I do to fix this? Is there a way to add a few more threads to the support rod? Any other suggestions?

Thanks!

Make sure the nuts on the bottom of the base plate are seated all the way.

They are seated as far in as they'll go. I'll double-check but I couldn't get them any deeper.