Lenovo N500 ram stick not recognized.

[summer]

Hi, hope for your help. I have an old laptop, Lenovo N500. it comes with a 2Gb ram module (ram1 the blue one in the pic) and it works fine. I wanted to add another 2Gb ram stick - (the ram2 pic, the green one) but unfortunately it's not recognized. even though it's installed, it shows only 2Gb on the "about" screen. what am I doing wrong?