Enrolled in graduate program for 2017 and not eligible for any tax credit, please help!

I am currently enrolled in Graduate program and was a graduate student in 2017 as well. I claimed American opportunity credit in 2015 tax return, but this year it seems that I am not eligible for any student credits. I paid almost over $13k last year in education and still no credit. I am filing return using Turbo tax deluxe and it says i am not eligible for tax credit. Any ideas?