If you happen to administer a large-scale system, you might not know what the system is actually doing. Managing hundreds of systems implies that you won't know what is going on with each of them.
The most important thing when it comes to troubleshooting boils down to a single word: data. If there is not enough data, there is no way to fix things. Therefore, the first step to troubleshooting is to always set up a monitoring tool, such as pgwatch2 (available at https://www.cybertec-postgresql.com/en/products/pgwatch2/), which gives you some insights into your database server.
Once the reporting tool has told you about a situation that is worth checking, it means it has been proven useful to approach the system in an organized way.