ElasticPress uses third-party software (Elasticsearch) and touches critical WordPress functionality that many other plugins interact with e.g. WP_Query. As such, instances arise where ElasticPress conflicts with another plugin, Elasticsearch setups aren't compatible with ElasticPress, and bugs are discovered. Use this guide to debug ElasticPress issues.
ElasticPress Debugging Add-On Plugin
The first thing to do when any ElasticPress issue occurs is to install the ElasticPress Debugging Add-On Plugin. This tool allows you to examine all the ElasticPress queries on each page load. It extends either the Debug Bar Plugin or Query Monitor, so you'll need one of those too.
Navigate to the page that isn't behaving as expected. For example, if a search page is returning no results, go to that page and open the debug bar. It will show the ElasticPress query.
To examine indexing, enable query logging in the ElasticPress Debugging Add-On admin settings page. This will keep a log of all queries that result in an error. Make sure not to leave this on too long as it can cause performance issues.
For a more in-depth guide on debugging, check out this blog post.