What version of 7.0 are you using? There was an issue with 7.0.4--so if it's 7.0.4 or before I would recommend upgrading to our most recent maintenance release of 7.0.14.
If this does not resolve the issue, I recommend sending the logs along with as much information as possible to our support team at email@example.com.
Thanks for the response Tracy.
We found the issue atlast and thank god it was outside of tableau. Some new firewall upgrade was closing the tableau server connection to database after 2 hrs.
The scheduled nightly extract was crossing 2 hrs and the manually kick started extract during morning hours was getting over in less than 2 hrs..!