Cannot remove/reduce prepared spell quantity
I am using the D&D Classic (2e) ruleset and have bought the full bundle so all of the books and several modules are loaded. For both Characters and NPCs, I can increase the number prepared next to a spell (right arrow), but cannot decrease the number. Casting the spell does not decrease the number either, nor can I type a 0 into the field. The only way to reset is to delete the spell from the character sheet and add it again.
I've watched the tutorial videos on this thinking I must be missing something obvious, like having to take a rest of something, but it looks like I should be able to toggle the numbers up and down when in Preparation mode. I thought perhaps something was broken on my installation, but my players connecting to me complain of the same thing. So now I'm really stumped. Please help!