Re-indexing through the admin site is a slow process but also "expensive". During the re-indexing the old index is being deleted to create the new one. While the re-index runs the ES is offline. If the admin closes the tab before it's completed the re-index pauses and the ES never gets back online.

Instead of suggesting to admins to re-index their ES every now and then it would make more sense to automate that process.

Tasks

• Investigate wp-cli options of ElasticPress plugin to figure out the less destructive one
• Add a post-deploy script to re-index on each deployment