XMP profiles are not guaranteed to work when using unmatched memory sticks. Since you're running two separate pairs of matched memory rather than one set of 4 matched sticks it's entirely possible that the memory will not run at that speed using the XMP profile settings.
Most of the time this can be solved by bumping up the DRAM voltage by .05 volts, try that
EDIT: The SPD contains a number of JEDEC profiles which are industry standardized speeds (typically 400,533,667 Mhz bus speed) so 1333MT/s (667Mhz) is the highest standard speed that will be automatically selected. The SPD also contains XMP profiles (typically 800, 900, 933, 1000, 1066) which are overclocked profiles that will not be selected by default