But instead of a total figure I was expecting, I got a 0 value.
I did a search online and the answer was the cells in the table could be stored as text instead of numbers. Since the original source file was done by a staff from my overseas office, I have no idea what she did. So for some unknown reason, the formula did not work as intended.
Here's what I did:
1) Notice the green triangles at the corner of the cells? Its actually trying to tell you something about those cells. What I did was to highlight the entire cells with the numbers and click on that "!" and select "Convert to Number".
2) The formula then worked at this point.
Thought I should blog about it for my future reference and to share with everyone. If you encounter this problem as well, why not give this a shot.