Manolo sling backs 95mm in black patent

I wanted some basic black slingbacks and bought these as I thought what could go wrong with Manolos?  Well in this case quite a lot.  My feet can't seem to stay in place when I have these on.  The centre of balance seems to be off.  The leather on the back of one of the slingbacks is also coming off after a couple of wears.  Disappointing as I certainly expected a bit more from Mr. Blahnik! True to size.