Most likely the problem is either at the last working fixture or at the first non-working one. If the wiring between these is indeed K&T, I would recommend that you call a (good) electrician. This is NOT a self help thing.

As a general rule, you troubleshoot by checking between the last working device and the first non-working device. If you are just blind guessing, the odds are that the circuit starts closer to the fuse box and ends farther away.