diploma not received after paying fees

Location/place: new delhi

Name of company/service: NIIT paschim vihar center

my name is sumit gaur. I m studying in NIIT. my first year computer course (DIT) was done in nov-dec 2011. now it’s june and i didn’t received my diploma yet. after calling or visiting my center, they give me 15-20 days time. after that the situation still remain same. they give me 21 days more. but it’s 5-6 months of my completed course i didn’t get my diploma. i have all payment receipts and all documents. pls help me.