Did you take them in all at once? I don't have an OD nearby, but I know Staples has a limit on how many emptys you can turn in within a certain period.. I wanna say there's a cap per day, as well as a cap per month, but don't quote me on that. They'll happily take all that you bring in at once, but anything above their limit doesn't net you the reward. maybe OD is the same way, although I'd have thought you would have received something in return for them.
Also wanted to know that our school has a fundraising partnership set up with
Green School Project that gives them money for any empty cartridges that they send. We just have to drop them off at the front office at the school and they send them in and get $ back. A quick phone call to your school will tell you if they participate in a program like that. Great way to support a local school if you're just tossing them anyways.