before you rma your ram...use cpu-z and see what the mb set the ram speed and timing up as. use the spd tab to see your ram speed and voltage spec. make sure with ram your not mixing two ram kits..most time they wont be the same voltage or speed.
if you have an intel mb and the ram rated for 1.65v check that the mb set the voltage right if not bump it up. also for some ram speed settings you have to use the xmp profile. if there is a error with memtest and you have two dimms. pull one dimm out and rerun..if it passes swap the dimm. if both pass. check that you have the ram in the right two slots. if memtest fails only with both dimms. see if there a bios update for your mb..the newer mb do have bios updates for cpu and ram code. when you swap the ram in your pc turn the pc off and unplug it..wait for the power supply to drain so you dont zap yourself.