I've read that the memory controller on the FX isn't that great so try to use only two sticks like 2x4/8gb's.It also isn't going to do well over 1866mhz ram because of that,you might try 2133,but that seems already high enough to go for.
With four sticks you probably would need to bring the speed down to keep it running,like with 1866->1600 for instance,maybe even to 1333mhz
If gunnerali got higher speed ram working is it up to you what to try,the official specs for the motherboard are up to 1866mhz,but that may be with four sticks.