Hi lesleyc,
I have encountered this problem a couple of times. Everytime this happens(in my case) it is because of the actual dialog box memory for studio spline going wild.
The way i have solved it is by going to preferences - user interface - Click Reset Dialog Memory - Restart NX.
This might not work for you, but a suggestion.
If this works for others or someone has another solution, please reply here for the benefit of other users.
Regards
L.K.