An effort by state attorneys general to take big mortgage servicers to task over faulty foreclosure practices has stalled as financial institutions demand broad legal immunity from other mortgage-related probes. The nationwide effort looking into faulty foreclosures, which involves attorneys general from all 50 states as well as some federal agencies, was expected to have produced a settlement of more than $20 billion by now. But talks have stumbled over how much the banks should pay as well as to what degree they will be released from liability from future investigations.