Each bank uses its own criteria. They also can come up with their own method of calculating the credit score. They don’t all use FICO. Debt to income ratio could be a factor, and the loan you took out is now showing on your cosigner’s credit as a new debt. (Since she is equally liable on the loan, it shows on her credit as if she were the borrower.) All of the banks have an appeal process. Work specifically with the bank that gave you the loan last year (if the rate and service are good) and have your cosigner contact them regarding the credit denial reason and the appeal process.