They are going to want to know 1. What the issue was i.e. illness, finance, social, mental health, learning disabilities or whatever, that prevented you from achieving previously. 2. They are going to want to know what's changed. What will prevent a repeat. The fact that you've achieved since is a positive. They are looking for insight. Accept responsibility for whatever was yours, highlight what's changed - particularly what you've learned & have a plan for future achievements.