Why didn't I get the full amount for the American opportunity tax credit?

It may have been reduced due to your modified adjusted gross income (MAGI).

According to the IRS, your MAGI is your adjusted gross income with the addition of the appropriate deductions, potentially including:  Student loan interest. One-half of self-employment tax. Qualified tuition expenses. Tuition and fees deduction.

To claim the full credit American Opportunity Credit, your modified adjusted gross income (MAGI) must be $80,000 or less ($160,000 or less for married filing jointly). You receive a reduced amount of the credit if your MAGI is over $80,000 but less than $90,000 (over $160,000 but less than $180,000 for married filing jointly).