Without filing a law suit, which would be costly, you have no way to force anyone to do anything. Your question is a little sparse in terms of the entity with whom you are dealing. But, you listed the debt in the bankruptcy schedules and so you knew what was owed when you filed, and you should be able to track your payments.
It may be that they are concerned that they would be in violation of the discharge order, which is rediculous since we all know that students loans are virtually impossible to discharge in BKC.
Why don't you have the co-signer to be the one to contact the creditor, which did not mind sending a coupon book to that person?
It sure beats having to pay money for a nonsensical law suit.