not recognizng additional RAM installed

I have Sony Vaio S570P model. which supports upto 2 GB RAM. My laptop came with preinstalled RAM 512mb. I have installed a sony compatible 1GB RAM in the extra slot provided . My notebook is not recognizing the new RAM installed. I tried removing / reinstalling , still doest work. There was no problem with RAM chip when tested at PCworld .
could any one please help.Thank you