Since you have mentioned that HR collected copies of mark-sheets and payslips, I assume you are working elsewhere.
I suggest you wait for sometime. Since its a Bank job, they will certainly do a background check before hiring. They need to verify your address, mark-sheets, past experience and all this consumes lot of time. Even if we assume that all procedures are completed and BGV is cleared, it can take some more time.
Let's say they have selected 100 people to be deployed in 10 new branches and if the approval for these branches gets delayed, then naturally positions will be on hold.
These are just assumptions, there could be some other reasons too. My suggestion is wait. Don't quit your present job, keep trying for other jobs also as this HDFC is getting delayed.