1. If you want to install two memory modules, for optimal compatibility
and reliability, it is recommended to install them in the slots of the
same color. In other words, install them either in the set of blue
slots (DDR3_A1 and DDR3_B1), or in the set of white slots (DDR3_
A2 and DDR3_B2).
2. If only one memory module or three memory modules are installed
in the DDR3 DIMM slots on this motherboard, it is unable to activate
the Dual Channel Memory Technology.
3. If a pair of memory modules is NOT installed in the same Dual
Channel, for example, installing a pair of memory modules in
DDR3_A1 and DDR3_A2, it is unable to activate the Dual Channel
Memory Technology .
4. It is not allowed to install a DDR or DDR2 memory module into
DDR3 slot; otherwise, this motherboard and DIMM may be damaged.
5. Some DDR3 1GB double-sided DIMMs with 16 chips may not work
on this motherboard. It is not recommended to install them on this
motherboard.
http://europe.asrock.com/downloadsite/manual/Z68 Pro3.pdf
Je nachdem was du für Ram hast. Evtl. auch Kompatiblitätsfehler. Ich würde das Board tauschen.