Fannie Mae's and Freddie Mac's effort to challenge the quality of the riskiest mortgage bonds they own is proving a tough slog despite the power of their federal regulator, according to sources close to the banks and regulator. Nothing has been heard from the regulator, the Federal Housing Finance Agency, on 64 subpoenas it issued banks in July for detailed information on subprime and other Wall Street mortgage bonds purchased by the U.S. home loan giants at the peak of the housing market.