One of the crazy aspects of RAM sticks is that you need the sticks to be identical. Since the old stick is unlikely to be found, try this. Using only the new stick, does it work? If not, it's the wrong stick.

If it does work, get one more of that stick. But that's usually not enough. There are over a dozen parameters to these sticks and I no longer do that work. But I do head to Crucial.,com to order 2 sticks and 99.99% of the time it works. (actually it's been 100%.)