Are you sure you enable nsfw on your profile? If you browse all, I think every 4th post is nsfw
My understanding is most NSFW posts are hidden by default unless you are logged in; if you are just visiting that instance directly and aren't logged in with an account there and changed your settings to view NSFW content it's gonna look pretty empty.
You should create a login specifically for lemmy.nsfw, there is a lot more there than you think and it's definitely growing.
The Community Search Tips community has the post you’re looking for: Finding NSFW Communities Using Lemmyverse.net
Lemmyverse.net allows you to filter by only NSFW communities, let's you sort by activity types, like subs, posts, comments etc. That's how I find new subs sometimes NSFW and SFW
Additionally, make sure to sort by hot or new, on NSFW communities especially top day won't show anythung because not all of them are posted to everyday
You may need to create a 2nd account on the lemmy.nsfw instance. Your instance may have defederated with them.
Check out lemmynsfw.com
You can also try the search from any of the instances, and so long as they’re federated with what you’re searching it should pop up.
I don't know why, but my instance seems to not index them for some reason. Some communities had some posts when visiting from my instance but after a few hours, all the posts disappeared and some NSFW communities have become empty. What could be the reason for this?