There are many factors than just you're current credit limit. Is your income up to date with Discover? There is a thread that focuses on secured Discover data points. It worth checking it out
http://ficoforums.myfico.com/t5/Credit-Cards/New-Discover-secured-graduation-policy/td-p/4784996
http://ficoforums.myfico.com/t5/Credit-Cards/Discover-graduation-data-points/m-p/4980304#M1455331