F9 calculate formula not working

When I press F9 its not re-calculating my formulas.

For example I have data in cell A1, B2 and in each cell beneath those two all the way down to A10, B10. In cell C1 down to 10, I have =A1-B1

However as I change the data, then try press F9 to get the new amounts, it doesn't update, however if I take cell C1 and copy it down, it calculates the new amount.

Anyone have an idea what might cause this?