I am waiting for answers to this one also. Once my Rowenta got gunk burned on from some non-cotton fabric in a charity top someone gave me to press, it hasn't been the same since. Twice my husband has helped me clean it with 0000 steel wool and a Dremel tool. It cleans up after lots of work and then gradually builds up again (in certain areas of the soleplate). I am wondering if the iron itself has "hot spots" that overheat.
P.S. The Rowenta cleaner doesn't do the job, but Nambe polish (with the steel wool and Dremel) works better. Not good, but better.