This can happen when your use tab is full. I could be wrong though.

Thanks Geylang — and chubby guessed right: your Use tab was full. The 3 Fruity Candy reward had nowhere to go, and instead of telling you, Chief Stan just hid the hand-in, so you only saw his letter line (and the Quest Complete! notice never popped for the same reason). Fixed: with all 100 ribbons, Stan now lists 'Halloween Day Candy Wrapping!' as ready, the Quest Complete! notice pops, and if your bag is full he tells you to make room instead of saying nothing. Your quest already went through after you sold those stacks, so you haven't lost anything.