I have had the same problem. The CD changer usually is not recognized but at random (and seldom) it may be recognized and operative.
You could try a canbus reset by disconnecting the battery, which can sometimes (though not necessarily) induce the changer to be recognized, but the problem in my experience will reoccur at some time subsequently.
My theory is that the problem may be related to the memory capacity in Blue and Me. If you are also using a USB drive in the glovebox for music that should be recognized but if it holds a lot of tracks, the system may run out of memory capacity. I say this because in my system not only is the CD changer not recognized. but not all the tracks on the USB (in my case >1,000) are recognized. This suggests memory capacity as a possible culprit.
I had this looked at several times by the dealer in Sydney. It is not a hardware problem to do with canbus wiring/connections or the changer itself. The dealer had no remedy other than battery resets.
In my case, I've given up on the changer and just use the recognized part of the USB, which works fine. Pity about the lost boot space.