Rules for assessing revised return.

In AY 20-21 I filed original return in time and three revised returns. The CPC assessed the 3rd (last) revised return and gave refund. After two months CPC assessed 2nd revised return and decided a demand. 2nd revised return was obviously having mistakes. I submitted responses and also grievances. All are unanswered. 

An officer from CPC  phoned me to pay the demand immediately. 

What are rules for such case?