Memory stick not recognized

[flarinator]

I'm trying to install a new ram module (Corsair 4gb DDR3 1600Mhz) on my Asus X555LN laptop, but it won't recognize it. This laptop has one 4gb stick built-in (soldered) and when I add the new one, windows still shows 4 gb.
What makes this so strange, that when I add the new stick and open a program like speccy or cpu-z, it recognizes the new stick - it shows the manufacturer (corsair) and all the necessary specs, but the total memory is still 4 gb.
Also, the memory slot on my motherboard has 1.5V written on it, and the new stick I bought is also 1.5V, however when I scan my system with Crucial memory compatibility tool it suggests me 1.35V memory sticks.
I've tried updating BIOS.
My operating system is 64 bit

 

[SinxarKnights]

You will need to switch to a 64-bit version of your operating system for it to show the extra 4GB of RAM. Windows 32-bit OSs are limited to 4GB of useable system RAM.

 

[flarinator]

I forgot to mention that I do, in fact, have a 64 bit operating system (Windows 8.1 64 bit)

 

[SinxarKnights]

Well from what I can tell it may simply be incompatible in some way. All the sources I can find all say you need 1.35V RAM modules. The 1.5V on the slot is most likely the max rating of the part itself, not what is compatible with your system. As in the socket assembly is rated for up to 1.5V applications safely.