Migration not found - D8
If your source plugin extends SqlBase, you need to make sure you have the database configuration set up correctly. Otherwise Migrate will silently ignore the migration.

In your settings.php or settings.local.php, add the following database configuration:

$databases['migrate']['default'] = array(
  'driver' =&gt; 'mysql',
  'database' =&gt; 'NAME OF YOUR SOURCE DB',
  'username' =&gt; 'YOUR DB USER',
  'password' =&gt; 'YOUR DB PASSWORD',
  'host' =&gt; 'localhost',  // change this as necessary
);


By default, Migrate looks for a DB connection named "migrate". If you use any other name, the migrations won't appear when you run drush migrate-status.

Note: The "migrate" connection is in addition to your "default" connection. You should have two connections in your settings if you're migrating from a MySQL database.

In case anyone's curious, the DB connection check occurs in the method SqlBase::getDatabase() in drupal/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php.

This is documented in https://www.drupal.org/node/2830031 and apparently will be fixed in Drupal 8.4.