Needs more specifics to provide a more detailed answer. mainboard, CPU, memory model numbers.
In some cases the mainboard will accept higher memory, but only runs at the lower speeds unless overclocked. You'd have to check your mainboard settings in the BIOS. Some boards are also sensitive to where you have to physically indicate the higher memory speed in the BIOS so it can apply appropriate voltages. It could be a safety measure to prevent damage to components. Your mainboard, may only support the lower speed, and doesn't matter what kind of higher speed memory you put in it.