How does indexing in Magento work?

If you are using the queue, it will only add items to the queue when they are updated. This is true even If the queue is set to update on schedule. Refer to the magento docs here and note the logic for partial re-indexing, which notes that the trigger for update on save and update on schedule is always updated data.

You can process the queue manually by running php path/to/magento/bin/magento indexer:reindex algolia_queue_runner . So, if you have just updated a few items, they will be added to the index after running this. 

If you have a number of items you want to re-index immediately, without waiting for the indexing queue, you can use the SKU re-indexing form. 

If you run php bin/magento indexer:reindex algolia_products when the indexing queue is on, it will not re-index all products. 

If you run php bin/magento indexer:reindex algolia_products when the indexing queue is off, it will re-index all products. 

Read more about indexing in Magento here.