American Opportunity Credit Eligibility for son

I am working on my tax return - MFJ - our son finished school in December with his associates degree.  He took an extra semester, so this would be his 3rd year for us to claim the credit.  Turbo Tax keeps saying he has claimed it 4 years.  If I change the data to say he's only claimed it 1 year (instead of 2), we get the credit.  What am I missing?