well it happens in CSS that people with higher education in a particular subject fall apart in this exam but i suggest you to identify what went wrong there, i myself have done MBA in Marketing, i don't know the institute you have done your BBA from but let me tell one thing that our GPA does not matter as the evaluation is not strict in our institutions and papers are also very easy but CSS is a competitive exam where you cannot expect such lenient marking as you enjoy back in your university...so identify the mistakes and come up with solid knowledge of the subject...