As per the Zigmeister from a previous post:
On the 7750, you have to press the release to get the stem to lock in place.
If you didn't press the release to get the stem to seat in place, but managed to get the stem to lock, then I suspect that you have damage to the keyless works.
I would remove the stem, the slowly and carefully put it back in place, turn it as you insert it, until it stops, then while pressing on the crown, press down on the release pin, to allow the stem to engage and lock in place. On some 7750's you have to press quite hard to get it to go back in place. You should feel and hear it click when it's seated.
Test it and see if it works. If not, then your left to take it apart, but on the 7750, you have to take a big bridge off to get at the keyless works, and getting this bridge back in place, is very difficult...
BTW how hard was it to get the hands on? I have that in front of me for yet another 6263 project....