Toshiba Satellite L755-S5151 will not hold memory chip?

Hello, my issue is I recently upgrades the OS on my toshiba satellite l755 s5151 from Windows 7 home premium to ultamate and noticed it running a bit slower. I have 4GB and bought a 4GB memory stick to upgrade it to 8GB. I did not realize that the 4GB I had was in fact two 2GB sticks. I removed both and installed the new 4GB in the first slot and then tried to install one of the 2GB sticks into the second slot but now it will not slide in. I can't see how the taps could have gotten damaged so my question is can the memory slots of this laptop be replaced at all?