After creating a Mastodon instance and adding some relays you may still seem to have empty feeds for a couple of reasons.
Follow The Hashtags
Hopefully you have added some hashtag specific relays rather than general relays.
Generating and using a hashtag relay
But if you want to see those hashtags and have them considered for trending hashtags to other users you will need to follow them with your user account too. This is because when you add a hashtag relay, it adds these posts to your content cache for you server, but it does not show your account what is in the content cache unless you have added it.
To see what your server is actually downloading search for the hashtags you have added to your relay, and subscribe to it yourself.
Is The Explore/Home Page The Problem?
When you start a Mastodon instance the explore tab or default home page may be constantly showing the message "Nothing is trending right now". This is due to the trending algorithm, read more about fixing this here.
Interact and Wait
Once you have followed hashtags and started interacting you will need to wait. This is because following things on Mastodon works like signing up to a mailing list. You need to wait for one of the accounts or hashtags to post or be used on the fediverse for your server to receive and show their content.

Running an Activity in the Pub is a great way to meet other people near you who are into the Fediverse

How to keep a secure copy of your YouTube channel on a PeerTube instance

When your Mastodon instance has a blank home page and explore tab that always says "Nothing is trending right now"