Cognizant Technology Solutions — No offer letter yet.. why?

I have given an interview for CTS in November.. after that I got a mail saying I have cleared the interview.. hr called me and said the same.. after a couple of days I got a mail requesting for documents from CTS .. I have sent the docs and a couple of days passed and I got a hr call for negotiations.. everything went fine .. but till now I haven’t recieved my offer letter. So requesting you to please look into this.