|The memory is incompatible. Could be the number or size of the chips on the module. Could be the timing or voltage. Doesn't really matter. That board supports dual channel memory. The modules must be exactly the same. Dual channel wasn't really an advantage with that series but thought I would throw it out there anyway.|
You could try installing them in the opposite order.
Consult your manual. There is mention of modules with over 18 chips are not supported.
Get your manual at the link below if you don't have a printed copy. Look at 2.5.1.