hmm I can only speak to the obvious; try each ram at a time, and try on different slots, one of the slots (but more often one of the actual ram cards themselves) might be damaged.
Other than that.. I guess make sure you've inserted it all the way (that's what she said), it shouldn't jiggle, it should fit snug (again, what she said) The pins should snap over the card, right where there is a semi-circle on both sides of the card.