I have the following crontab entry to run a shell script for every 30 minutes of every day:

30 * * * * $HOME/main.sh > $HOME/main.log 2>$HOME/error.log

after I created the crontab file I have also done:
$crontab my_crontab

I also check to make sure it exists, by using the following command.
$crontab -l
I get the crontab entry- 30 * * * * $HOME/main.sh > $HOME/main.log 2>$HOME/error.log

But, why is the shell script not getting executed every minute?

please read the rules:
(5) Search the forums database with your keywords before asking.

I actually searched the forum and read the cron and crontab article. I did not find a resolution to this problem. Which is why I posted it.

in this case you've probably read about the enviroment settings as they pertained to cron?

I didn't think the problem was in environment variables- because the following works:
45 13 * * 1-5 $HOME/main_ics.sh >> $HOME/main_ics.log 2>>$HOME/error.log

but not when I change the cron to:
30 * * * * $HOME/main.sh > $HOME/main.log 2>$HOME/error.log

aren't they 2 different scripts?

They are not I just changed the script name for posting and forgot to change on my reply.