No theme applied if I use subdomain name but proper theme applied if I use IP Address

I have a very peculiar problem with my setup and don't know what the issue is.

Up till now, I've been accessing my Drupal site by it's IP address as at that time we haven't set up a domain or subdomain.

I am using:

• SimpleNews - 7.x-1.1+0-dev
• HTML Mail - 7.x-2.65
• MailSystem - 7.x-3.0-alpha1
• Elysia Cron - 7.x-2.1+9-dev

and I have configured HTML Mail to use my specific newsletter theme.

I've also setup a crontab to call my Elysia Cron:

*/5 * * * * wget -q -O /tmp/cron.txt http://[IP ADDRESS]/cron.php?cron_key=[CRON KEY]

This works fine and I receive the newsletter with the proper theme.

I now switched to use a subdomain to access my site and made modifications to the crontab but now, when I send my newsletter, no theme is applied!

If I switch back to use my IP address, the theme is properly applied again.

I have no idea what may be the problem. Would anyone know what could be the problem? From what I can see, no theme is actually applied at all even though I have my theme set in HTML Mail.

Thank you for your help.