Man, you got some copying issues. When you put a formula in row FOUR, make sure the references IN the formula are looking across row FOUR. You have the exact same formula in AX4 and AX5 which is looking at row FIVE, which is blank. So the answer is zero.

The formula is working. Make sure the formulas you are "analyzing" are appropriately entered for the row.

Copy the CELL AX5 up over AX4 and it will adjust the formula for you.