sometimes hashtags ebb and flow. Concepts like environmental sustainability are quite normal, so aren't necessarily tagged. You may have to branch out what tags you're following.
It's a good idea to look at what other tags are in use on the posts you do see and appreciate, and follow some of them as well. For example some "sustainability" posts are also tagged "solarpunk" or they may have more specific tags like "activetransport".
Looking through the timelines of people who post and boost things you're interested in is another good way to find the sort of the you want.
