Unfortunately, you are doing everything right, and just as unfortunately, OPM is responding as they have been in recent times. Your persistence is what will ultimately prevail. The only thing that I might add is that, you may want to have ready and on hand a recent medical report/narrative showing that you are still disabled from your job, and your conditions (have the doctor list them) continue to prevent you from performing your former job. They may well tell you, after all this time, that they don't have that on file, and that is the "reason" for the delay. You should likely preemptively obtain it and forward it to them as another piece of the paper trail. Keep pushing and call as often as possible to try and get a new case worker assigned.
Sincerely, Robert R. McGill, Esquire