It is fine to use memory under it's rated level. Most memory modules come right off of the same assembly line, and are then binned for specific useage depending on what the chips will tollerate. The better chips will go into high spec units, and the worse ones will go in low spec units, but technically it is all the same stuff inside. You can always run slower, it is the running faster that tends to cause problems.
I would look out for voltages. Some of the new ram is very low voltage, just make sure that it matches what your mobo will put out.
If the memory is not wanting to work at the spec that it is supposed to then I would go for an RMA as it may have other issues.