Mine has the same issue with the head. It sits on the figure instead of clicking in securely. Looks fine in museum poses but makes flying poses difficult.
Still one of the few figures I can't ever imagine selling. Even the box is awesome and I don't usually care about packaging. My only wish apart from a better cape is that it had come with a second sculpt. Would have loved a smiling Reeve sculpt. All HT Superman figures have that very stern look neglecting an essential part of the character.
Sounds like you might of cracked the neck adapter, this has happened to me on a few figs, fortunately they sell them on ebay now, so you can replace broken one.
Mine has that cracked neck piece. I got mine new, never took the head off, and one day it just went plop. Head just feel to the floor. It still works mostly fine for me, even in flying poses, so I haven't put effort into replacing it, but it shouldn't have happened period for a piece that I never stressed.