Asus Vivobook a407m Ram upgrade - detect 8gb ram as 4gb

Hi, I have an asus vivobook a407m. Originally it comes with 4GB DDR4 2400MHz ram with only one slot available. i searched online and found that the ram is upgrade-able to 8GB ram. I even run my laptop with Crucial System Scanner and it suggested the same. So I bought an 8GB 2666MHz ram online and installed it myself. but once installed, in the bios, the system, and task manager shows that it is only 4GB 2400mhz (3.83gb usable) I thought i was scammed buying the ram online, but when i insert that ram to another laptop, it is genuinely 8gb ram. just in my laptop it shows 4GB. Why is that? and can anyone help me to make the system read it as 8gb ram? or should i change it to 8gb with the same speed of 2400mhz? Thank you in advanced.