If it's got 4 RAM slots, it may need all 4 populated. Sometimes if you only use 3 the mobo 'ignores' the 3rd stick. Try borrowing a 4th stick and see if you can get 16GB before you return it.
Although yeah an i5 with 16/12 GB is like putting chrome 22's on a car worth $5k.
Although yeah an i5 with 16/12 GB is like putting chrome 22's on a car worth $5k.