Problems fitting new RAM?

Hi there, my laptop was running slow so I oredered some new RAM. My laptop is an advent 5421, it has 1 GB of RAM, I ordered a 2 GB module and it arrived today. So when I went to fit it, I have to take out the old memory as there is only one slot. The new one fits in perfectly and securely, but when I turn the laptop on it just wont boot up, I tried it over and over again, but no luck. And when I put the old memory in the laptop can put up no problem. Has anyone any advice for me? Am I doing something wrong? Kind regards.