The key to rebates is they want people who value their time more than money to not bother jumping through the hoops. The key to dirty rebates is it's not worth the time to try to process a claim when they "forget." Of course all companies are going to lose a few rebates, but indeed there are things to suggest some companies are too forgetful.

You tried the first step, to complain, which often is enough. (They just figure that only a fraction will complain.) If that doesn't work, you can try the attorney general. Alas, if you didn't document what you sent (a very easy way is to take digital camera photos of the whole process) you won't be able to do much. The AG however can notice that a lot of complaints of "didn't get anything" are coming in, too many to be accident, and act on it if so. Not much hope for you.