Visa nit stamped and passport returned

Hi,
I applied for renewal of C1/D VISA through drop box in mumbai. Later i received my passport without visa been stamped and no reason for rejection.
I had never been rejected any visa and few months back i applied for B1/B2 and i got it as well.
Is there any particular reason for not stamping and where can i mail or call to know the reason.
Thanks