If your American Opportunity Credit is phased out and you are not getting the full amount of the credit, then it may be due to your modified adjusted gross income (MAGI).
In order to receive the full credit amounts, your 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).