We would recommend submitting this as a ticket so that we can do a specific investigation into this use case. Some potential causes could be:
1.) A delay in calculation due to a major change like adjusting someone's rate (check after an hour or two to see if the field has calculated)
2.) A workflow rule that is forcing this amount to be 0.
3.) It could have been accidentally set to 0 (look for a blue triangle in the upper right of the cell, click the cell and then the "restore" link to revert back to system calculations)