In your Spring properties, set endpoints.health.sensitive = false. The /health endpoint will then return the list of various health indicators and you can debug from there.
For a production environment you should enable security around the /health endpoint.
Edit
As Vincent pointed out below, you’ll also need management.security.enabled = false if the health endpoint is secured, which seems to be the default in more recent versions of Spring Boot.
A common issue that I’ve seen with Spring Boot out of the box is that it auto-configures Solr, and without additional configuration the /health endpoint indicates that Solr is DOWN. An easy way to fix this is to disable the Solr auto configuration in your Application.java with this annotation:
@SpringBootApplication(exclude={SolrAutoConfiguration.class})