You are correct that if Education Expenses exceed Scholarships/Grants, you are eligible for an Education Credit, although there are income limitations.
Since we can't see your tax return in this forum, you may want to check your entries in the Education Section. There is no program issue regarding the AOC.
Make sure you didn't accidentally indicate that your student has claimed the AOC four years previously. This question is worded so that you may want to choose YES instead of NO.
If you can't resolve your issue and believe you should qualify for the AOC, click this link for info on How to Get a Live Review.
This link gives details on qualifying for Education Tax Credits.