Yep, most likely one stick of ram has memory chips only populated on one side of its board.
Where as the other stick when you look at it will have memory chips on both sides of the board.
Some memory sticks can come in different sizes or capacity of memory block for each chip on the board.
And boards don`t like it, when mixed.
I suspect it`s the case even if the memory speed matches, and the latency timing values of each of the sticks. By the way the timing values are very important.
If it differs from one stick to another then it can be a problem.
What I think you may find is, it will depend on the order you put the memory stick in the slots also.
Also Voltage of each stick, must be the same.
Take a look. It may just be one dead stick of memory though.
I forgot to say Don`t forget to reset the bios back to factory defaults also.
As the auto detection of the last set of ram sticks being auto detected by the bios may still be stored.
And causing an error with the new memory based on prior settings stored in the bios and not being refreshed right by the bios