If you are using the indexing queue, Magento 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 please note the logic for partial re-indexing: the trigger for "update on save" and "update on schedule" is always updated data.
You can process the indexing queue manually by running php path/to/magento/bin/magento indexer:reindex algolia_queue_runner. If you have only updated a few items, they will be added to the index after running this command.
If you have a number of items that 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 when you run that command. You will also need to additionally run the indexer for your indexing queue if you want to re-index.
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.