_csvRead function Jmeter not reading custom column/rows

I am writing Jmeter tests to read data from multiple CSVs to create more data driven tests.

I would like to read data from 2nd column and multiple rows in it. Column contains 2 different time stamps and I am trying to calculate the difference between specific two specific rows on specific column.

For example, I want to read this csv and perform action like : Variable= value_of Column3b - valueofcolumn2b. This calculation should be performed with consecutive row value.

While reading csv by : ${__CSVRead(whatever.csv,next)}${__CSVRead(whatever.csv,next)}, I am getting null value.

Tests are running on Jmeter 5.3. Any idea what is the cause of this and how to fix it? Also, how can I perform calculations in returned values?