That could be a faulty in one of the DIMM Slots, but as the 4 slots works with the old ram, then one of the G.Skill stick might be faulty.
In order to find out if one of the new sticks is faulty, you have to test them separated -- one at time -- if you find one that does not work, then you may have to return the whole kit to the manufacturer -- just RMA it.
Both RAM work seperately very well. I asked the MSI forum and they told me to better use Corsair. No OCZ nor G.Skill and that 1600 is best for performance. any higher is not remarkable. can someone confirm that ?