Unlike Twitter, hashtags don’t perform a global search, they only perform a local search on the content that people have pulled into your instance via subscriptions; this is a downside of it’s federated nature. So what you are finding out is essentially that people on your instance don’t share your interests.
If you want to improve your feed, you should look for instances where people who are interested in the same kinds of things as you congregate, and subscribe to the people there who interest you. If you find an instance whose community really clicks with you, you might consider switching to it, and then the hashtags will work better for you.
In general, it helps to model the fediverse as being not one community but a big community made up of a bunch of smaller communities that all talk to each other, so it’s more like a Twitter alternative than a Twitter replacement (even though it is sometimes sold as the later rather than the former). Personally, I find Mastodon to be infinitely better than Twitter, but that’s just because I personally never used Twitter due to lack of interest so I don’t have a basis for comparison. :-)