|Download SIW and use it to check the specs on the RAM you have installed. |
You don't have to have matched pairs to make them work. When a motherboard has Dual Channel capability then it may be necessary to have the same exact modules.
I think that for whatever reason, crucial came up with the wrong RAM or you didn't order the EXACT module they recommended.
Get SIW at the link below. If the specs of the RAM are different then install the slower RAM in the first slot.