It may be because you have no tax liability for the year.
Look at line 41 (Form 1040) or line 25 (Form 1040A). If that number is zero, then you don't have anything left to subtract.
To look:
1.
Go to Tax Tools on the left-hand side.
2.
Go to Tools.
3.
Go to View Tax Summary.
4.
Click Preview my 1040 from the left-hand
side.
