When I got my vehicle the glow plugs were shot, and the EESS box was "fine". I replaced the glow plugs, then found out actually the box was bad. Replaced the temp sensor on the engine (that connects to the box) and the box.. and now it was starting great all the way down to about 40F. Below that you can hear it trying to start, but it died right away. Turned out the temp sensor on the side of the engine that kicked in the low-temp solenoid on the injection pump was bad. Replaced that, and it was much smoother down to freezing. (Unfortunately due to other problems, haven't been able to run it below freezing yet.)
So take it one piece at a time. If your glow plugs are "new", verify they're still good (as mentioned before). Then focus on if they're getitng the voltage they should be for the time they should be. Then verify the temp sensor on the side of the engine...