How do I debug my Feeds import not importing a field
As a first step, you could check whether the field has been parsed properly:

In sites/all/modules/feeds/plugins/FeedsProcessor.inc:

Add debug($item) after line 180:

  public function process(FeedsSource $source, FeedsParserResult $parser_result) {
    $state = $source-&gt;state(FEEDS_PROCESS);

    while ($item = $parser_result-&gt;shiftItem()) {
       debug($item); // debug entry

      // Check if this item already exists.
      $entity_id = $this-&gt;existingEntityId($source, $parser_result);
     ...


debug() is a core function that lets you print out debugging information:

https://api.drupal.org/api/drupal/includes!common.inc/function/debug/7 and/or

http://blog.boombatower.com/drupal-7-debug-and-simpletest-verbose?utm_source=buffer&amp;utm_campaign=Buffer&amp;utm_content=buffer5ee6f&amp;utm_medium=twitter

The result will be processed using drupal_set_message(), and will appear in http://example.com/admin/reports/dblog (probably, depending on your configuration). 

You'll get one notice for each imported line, like:

User notice:

array (
  'title' =&gt; 'This is a test title',
  'body' =&gt; 'And a test body',
  'tax' =&gt; 'And a taxonomy term',
  '' =&gt; '',
)


Alternative: download (from http://drupal.org/project/devel, or drush dl devel) and enable the Development module (Devel in admin/modules), or drush en devel and use drupal_debug() or another tool from the devel module.