The SUM function in Numbers 3.6.1 on my Mac OS X10.10.5 isn't calculating correctly. Why?
If the cells are in text format, they won't be counted. Check they are a number format cell first.

EDIT
I have tried it myself, when you find and replace, the cell is not automatically updated to a number cell. If you manually type in the number 1, it automatically gets changed to a number format.