I can't put my computer to sleep

I can't sleep my laptop as the screen goes black every time I sleep it. The disk drive indicator doesn't flash and the screen indicator is still on (my laptop has 5 indicators: screen, power, disk, network and caps lock). I use an ASUS X44H. I can't turn my laptop on so I have to hold the power button and start it up again. This problem didn't exist before I replace the stock 320GB HDD with the new 520GB HDD. Is this because of my new disk? How can I fix this? Thanks a lot.