without details on your mobo and the ram, it's impossible to pinpoint a single reason. it could be any of the following:
* bad ram slot
* spd is misread (or incorrect) and an incompatible timing is chosen. (if so, just manually set the highest timings in the bios.)
* some mobos can handle single+double sided or two double-sided sticks
is there ever a common reason why 2 rams sticks will not work together, yet work fine independantly in the same machine?