Cognizant Technology Solutions — regarding delay in offer letter for more than 3 months

I attended cts interview for bigdata on 10 jan 2019. I got selected and they told i will receive offer letter very next day.

On 15 i received cis form to be filled by cts. After that no update, again on 29 mar they again sent cis form

I am not sure why they do like this. Please help on this

Spoc;jayashree karunanidhi
Hr:neil dyson and sakshi