RAM typically contains a number of performance profiles stored in an EEPROM on the DIMM itself, allowing the stick to be "backwards compatible" so to speak. In a perfect world this would always allow memory to function in a computer, alas, that is not the case.
Your computer may decide upon finding higher performance profiles than it supports, to not like the memory and then wonk out. Or it is simply incompatible with your system. DDR3 RAM is meant to be standardized, but in practice one stick may not work in one system but will in another.
Often manufactuers of PC motherboards have lists of certified memory that has been tested and verified to work with each product which you can look up on their website; I don't know how Apple does this however.
Usually the best idea is to use only identical modules, and not mix RAM of different models, sizes, speeds, timings or from different manufacturers.
If you have access to another laptop you could try the RAM in that one, see if it works. Otherwise you may want to contact your place of purchase.