Fannie Mae and Freddie Mac are facing growing resistance as they attempt to push failed home loans off their books and onto the balance sheets of banks including Bank of America Corp. and JPMorgan Chase & Co. The two government-owned mortgage companies are enforcing contracts that require lenders to buy back loans that didn’t meet underwriting standards. At the end of September, the companies reported, banks hadn’t responded to $13 billion in buyback requests. A third of those were at least four months old and Freddie Mac has begun to assess penalties for the delays.