Here's a couple of things to try out. In your bios, make sure that the memory speed is set at 1600MHz if that is what your memory speed is. Set is manually if you have to. If that's not the case, try this:
Click start, in the box type, msconfig, and press enter.
click the boot tab, ten click the advanced options button.
In that window there is a setting for Maximum Memory. Make sure the checkbox is NOT checked, then reboot your computer.
Other than that, just make sure that it is properly installed in the slots, maybe remove it and put it back in again.