There is a special controller action to assist with debugging your feed, should you encounter issues, or errors during an import. The easiest way to get to this is clicking on the small 'bug' icon on the feeds index page (image below). The icon is placed in the Actions column on the far right, but only visible when devMode is turned on.
The output from this page will be in raw format, providing you with as much information as possible about your feed settings, field-mappings and data. If there are code exceptions, they'll appear on this page.
Available for Craft 2 & Craft 3, with Free and Pro versions.