One of the pillars of the consulting side of the work we do here at DrupalEasy is data migration into Drupal sites. Over the past few years, we've been focused on migrating data into Drupal 8 using the most excellent core migrate modules along with contrib modules like Migrate Tools, Migrate Plus, and Migrate Source CSV.
If there's one thing I've learned over the years is that data to be migrated is never, ever, ever, ever, never as clean as it is claimed to be. There's always something that has to be massaged during the migration process in order to get things working.
One of the most common things I see when migrating data from a .csv is strings with trailing spaces. If you take a cursory look at the data in a spreadsheet, you might see something like "Bread", but if you look at the same data in a text .csv file, you'll see that the string is actually "Bread " (trailing space). If you're migrating this field to a vocabulary using the entity_lookup process plugin, that trailing space will cause the term to not be found, and therefore not migrated.
The solution? Well, you could clean up the data, but there's actually a much easier solution that I use by default on almost all string data being migrated - I use the "callback" plugin to in-turn call the PHP trim() function on incoming strings in the "process" section of the migration configuration. Here's an example:
field_topics: - plugin: callback callable: trim source: Topic - plugin: entity_lookup entity_type: taxonomy_term bundle: topics bundle_key: vid value_key: name ignore_case: true
Using this method allows for the incoming data to be a little dirty without affecting the migration.