I vaguely recall scratching my head on this one before. Try using the Unlimited Spell Scrolls Usage option. I tend to toggle on all 3 of those options (NUM ., F2, F1) and that definitely works with my Wizard's spells. Mousing over the portraits at the bottom of the screen after hitting a key has worked well for me.
I've also found it's one of the first features to break if the devs "fix" something todo with savegame corruption. Just guessing that that meddles with the memory locations things are at.