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When you add the second RAM do you get any blink lights? 3 blinks on the LED close to the power button it means memory issue, so your RAM could be either defective or not compatible.
You can see the RAMs tested with this unit at:
Make sure you have the RAMs fully seated on the slot.
Remove and reseat the memory.
Make sure the contacts on the memory and the socket are clean.
Check for a faulty memory module by trying the memory in a known good system.
Try using just one memory module at a time, swapping it between the upper and lower memory slots.
Try using memory modules from the same manufacturer with the same part number and speed.
Test with different RAM if possible, especially with one of the units mention on the link above.
You can also try a BIOS update; you can download the latest bios version for this unit at the following link:
Please follow the steps to do a BIOS update here: