Maybe the sections got mixed up, the 2 thread sections, the ones that press into each end of the bottom barrel have different thread lengths...the one with short threads goes on top for the nib, longer threads one on bottom to post cap.
Angela, if your question is referring to the feather pen you posted prior to this, you have the 2 threaded couplers mixed, if it matters to you if the cap can be posted you'll need to switch the couplers.
I was aware of this because I had already did this on my first George

..I just left it the way it was, the pen was for me so I didn't care about posting the cap.