1. What is a P/N of the Crucial module you have bought? Please attach an image of both sides of this module.
2. If you have 2GB module that have total 4 chips (i.e. it is build from 4 x 512M chips), the density is 512 X 8 = 4096Gbit or 4Gbit - which is the compatible density.
3. Here you can find checked by Intel memory modules: http://compatibleproducts.intel.com/ProductDetails/ExportPeripheralInfo?moduleName=Intel%C2%AE%20NUC&productType=Mini%20…
That what I was suspecting. You have incompatible memory module with 2Gbit density. My advise is to buy memory listed in Intel table. My other suggestion is to buy 2 module kit. Installing in the free slot a memory module from other manufacture as the existing module, will cause that the system will not work in double channel mode and therefore will loose in performance. Even installing second module with the same P/N doesn't ensure that it will work in double channel mode. Therefore a kit of two modules (exactly with the same specifications and performance) should be installed to gain full NUC performance. In the following link you may found an example of such kit: Crucial 8GB Kit (4GBx2) DDR3/DDR3L 1600 MT/s (PC3L-12800) Unbuffered SODIMM 204-Pin Memory - CT2KIT51264BF160BJ at Amazo…
1. 2X 4GB - CT2KIT51264BF160BJ (be sure not to order CT2KIT51264BF160B - similar P/N without "J" in the end. This module has incompatible density)
2. If you prefer 4GB single module CT51264BF160BJ is compatible.