Products: Nintex Workflow 2007, Nintex Workflow 2010, Nintex Workflow 2013
Workflows do not start automatically due to a SharePoint security fix.
After you install Windows SharePoint Services 3.0 Service Pack 1, workflows no longer run automatically when you are logged in to SharePoint as the system account.
A security fix in Windows SharePoint Services 3.0 SP1 prevents declarative workflows from starting automatically under the system account.
Set the application pool account to use a different user account. Refer to the following Microsoft knowledge base article for more information: http://support.microsoft.com/kb/947284/en-us
PLEASE NOTE: While this post mentions SharePoint 2007, the behaviour introduced at that time has persisted from SharePoint 2007 right through to SharePoint 2013.
For information about a similar issue in Windows SharePoint Services 3.0 related to starting workflows by emailing document libraries, see the following Microsoft article: http://support.microsoft.com/default.aspx?scid=kb;EN-US;953289.
Note: For SharePoint 2010, perform an IISRESET after running the stsadm command referenced in the above article.