I tried to kill a process using pkill -9 <pid>

Whenever I run that command it returns nothing and the process isn't killed either.

How do I proceed now?

Edit:

I was stupid trying to use pkill with pid. pkill takes the name of the process not pid. kill is the command that takes pid as pointed out in the accepted answer.

However the other answer actual does a good job explaining the actual possibility of pkill not killing and is very well written.