It could be some FOD stuck in the slot. You could also have a bad slot (a friend has a unibody MBP with such an issue, only 1 memory slot usable).
Many years ago I learnt a valuable lesson through causing a small but intense fire by putting RAM that was too new in a slot that was too old... once the smoke cleared the damage was virtually unnoticable to the untrained eye, but the slot was (literally, to a certain extent) toast.
