I got the same problem on the LHS mirror. It will fold back but will not stay folded back. Dealer said it was a buggered/broken micro switch and require the whole mirror to be replaced.
If it is not buggered, you may be able to do a bit of adjustment and it will be good. Push the mirror manually back a bit and it will become wobbly but press the button to fold the mirror and open back again, it will stay firm.