I have to agree with Mike. My problem was directly related to the glue residue inside the nib end of the tube. I'd probably also argue that residual glue inside the clip end attributed to my secondary problem, which I'll explain in just a minute.
The instructions said to screw the transmission into the nib, then press the entire pen together. Once I did that, I could not unscrew the nib from the transmission because the residual glue made the fit too tight.
It took three people and a couple of those rubber jar openers to completely unscrew the pen. Once apart I cleaned all the residual glue from inside the nib end of the tube. At that point, I could thread the parts together and apart without issue, but the twisting action was rough and retracting the tip resulted in unscrewing the pen.
Initially, I thought I just hadn't properly tightened the threads, but it turned out that this was not the case. I completely disassembled the pen and cleaned the residual glue from the clip end of the tube. Once reassembled the kit worked as expected. Twisting was smooth in both directions and the pen no longer came apart when retracting the pen.
I don't know all the technical stuff, but I believe (in my case) that residual glue in both ends of the tube resulted in two different problems.
Sometimes I just get excited and forget to clean the glue out of the tubes or forget to square the ends of the blank to remove finish residue prior to assembly. I probably won't be forgetting these steps in the future.
Paul