Memory speed not correct

I don't know if this should be posted here. But I really need some help. If I should post this anywhere else, please let me know.

I rencetlly updated my 1600MHz 8G memory to 2400 MHz 16G memory in my Z97N motherboard. Of course the memory is DDR3. But after enabling X.M.P. in BIOS, the memory frequency still freeze at 1333MHz.
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But the weird thing is the M.I.T. Current Status in BIOS shows the frequency...

Memory speed not correct