Memory generally comes with conservative timing recommendations in the SPD (Serial Presence Detect) ROM, to ensure basic functionality on all systems. If you install a 1866MHz speed kit, it doesn't run at that speed automatically.
A few things to remember about setting up memory.
1) Memory Clock
2) DRAM Timings
3) DRAM Voltage
You can set these manually or enable an eXtreme Memory Profile (XMP) (page 38 of mobo rev.3 manual).
XMP profiles are commonly used as it sets all three parameters listed above to the rated specifications.
XMP profiles are [Disabled] by default in the BIOS.