Hmm...I had the same problem. My RFE was issued Jan 29th, and I had moved, switched companies, switched lawyers (subitted G-28 for substitution of attorney), and the RFE was sent to my old lawyer, and I didn't get it. I called USCIS and got the address changed, and asked them to reissue the RFE, they did, but apparently they sent it again to the old lawyer (as I learnt later by calling them again). So after about 30 days I called again, and asked them to reissue it again, and I specifically asked them to send it to my home address (We had also resubmitted our G-28s for subsitution of attorney again).
Meanwhile, I was able to track down my RFE through the old attorney, and got it from them. So finally we were able to file a reply pretty just in time before the 90 day deadline (I think). A few weeks later I got a letter from USCIS that they had received my query to be issued an duplicate RFE but since they had already received my response they will not be sending it.
Well, I'd say stay at it, keep calling USCIS after the expected time to receive the RFE has passed. You should be able to get it. The frustrating part is, only if they would tell you what the RFE is about, you could atleast prepeare a response.
Still I would suggest, get your materials ready for a standard RFE. Routine ones are - Employmen letter, paystubs, tax returns etc., so you may go ahead and assemble it all now. Ofcourse the RFE could end up to be about the Medical (as was in my case, though they still asked for employment letter etc.).
Also if you think it was lost in the mail because you changed lawyers, or your lawyer changed addresses, see if you can track it down to where it was sent to. It is always sent to the lawyer, no copy is sent to you.
Well, good luck. I got my approval pretty quickly after the RFE, so look at it as a good sign, this may be the last hurdle.
Cheers!
