Yes, often it's the locked padlock on the grade item that prevents students (and teachers accessing the User reports) from seeing grades..
I think locked gradebook items should **really** by default be labelled more visibly in the gradebook - like with a different background colour etc (red ?) - it is so easy to miss them. The padlock is displayed way to the right, and I have missed it several times in the recent days. One of my scenarios I restore courses from previous academic years into the current academic year - in the from-course I did different things, including locking of gradebook items. After restore / import into a to-course I just forget about the locked items, assuming the gradebook, carefully crafted for the old course, will just work..
This response would be more suitable on the gradebook forum, I know, but I thought I'd share it here as well.
Anyway, the link given was very useful.
Przemek