which motherboard did you insert it into? For instance, is it an older motherboard that takes DDR2 and DDR3 ram? If that's the case then it won't work? Additionally, even mixing and matching various types and speeds of all DDR3 ram may or.. may not work.. it's just random if you're lucky or not... Some people can mix sets and have a working computer... other people may end up with glitches and crashes.. others may not even boot up..
In any event, a little more information would be helpful... which motherboard, which processor, and tell us about each of the ram sticks you're trying to use please.