Did you read the Notice To Co-Signer being produced? If it follows the model form I'll bet you'll find a sentence that reads, "This notice is not the contract that makes you liable for the debt."
Unless you have a very unusual and legally binding Notice to Cosigner Form then most likely none of your loans that you thought had cosigners do because you most likely did not have them sign any legally binding contract.
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The opinions expressed are mine and they are not to be taken as legal advice.