Why am i getting General HTTP error: Domain name not found ?

Google webmaster tools is giving me the following error message for a number of my channels:

General HTTP error: Domain name not found
but the sites are workingGoogle webmaster tools is giving me the following error message for a number of my channels:

General HTTP error: Domain name not found
We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.

The channels are:
seenjeem.maktoob.com
cars.maktoob.com
quran.maktoob.com
dreams.maktoob.com
daleel.sh3bwah.maktoob.com
ramadan.maktoob.com

Our Google Search traffic has dropped, and many pages are not indexed or cached in Google after that issue.

We have checked our DNS, and everything is working fine.