When publishing a new Nintex Form, only the "Edit" and "Display" forms are updated. The new item Form renders as an out-of-the-box SharePoint Form; deleting and republishing the form does not replace the NewForm.aspx page.
Rebuild default List .aspx files using PowerShell. For more information, see Recreating default display, edit and new forms of a list in SharePoint 2010 using Powershell in Microsoft TechNet.
This occurs when the default SharePoint "NewForm.aspx" has been deleted through SharePoint Designer, even if the file is recreated it will still not be picked up and replaced by Nintex when publishing a new Form.