The way hashtags work on here is they filter whatever accounts/posts are visible to your server anyway. So, you only see tagged posts from accounts that your server has noticed.

Users can avoid this problem by using "groups", where every post is sent to everyone in the group no matter which server they are on. More info at https://fedi.tips/how-to-use-groups-on-the-fediverse

Admins can subscribe to hashtag relays to increase posts for tags visible on their server, more info at https://fedi.tips/using-relays-to-quickly-expand-a-servers-view-of-the-fediverse/

Could it be there are current posts using them but my instance can't reach them?

Relays are perhaps quite a blunt instrument to deal with a particular account looking blank as they take up so many resources? Following the account is the most efficient way of getting it to show posts on your server.

By the way, there's a fix on the way which would also show an account's previous posts as soon as the follow happens (aka "backfilling"), it's already written but waiting for review: http://github.com/mastodon/mastodon/pull/34597