Can anybody please tell, why only HDFCBANK May future is trading at discount as compared to April and June Future ?
Due to dividend most likely. Company announced 19.5 Rs. dividend with May 10th as record date
In that case, It is direct loss of around 10Rs for May future holders, and we don’t get the dividend also 
It should also be cut in Jun Future, that is beyond that record date, no ?
You also have to add the risk free interest rate to the premium.
Check this article - The Futures Pricing – Varsity by Zerodha