RAM modules are difficult to install on this motherboard as there are only one locking clips on one side of the RAM slots. Therefore, make sure that you apply force on both upper edges of RAM sticks equally while seating them.
If this does not resolve the issue you are experiencing, enable D.O.C.P. in BIOS and try again. Then enable X.M.P. in BIOS and try again. Do not forget to save settings before you exit BIOS. Otherwise, changes will not take effect. You may also consider updating BIOS if you have not done so yet.
If all of the above attempts do not help, please be advised that some people resolved the issue by removing the CPU, checking the processor and the cpu socket for bent/uncontacting pins. Since the memory controller is in the CPU die, not separate as in older generation of Intel CPUs ( Northbridge-Southbridge thing ), CPU installation is critical.
Hope this helps.