If I see a "sub" in my feed I can click on it, which takes me to the sub. From there tapping "subscribe" works just fine.
But I can only search for new subs via the web (browse.feddit). Clicking a link there takes me to Jerboa, but not to the sub. Somehow I just can't navigate to a new sub via a weblink, or any other way for that matter, unless that sub just happens to appear in my feed.
I noticed that search for subs in Jerboa is slow and somewhat unreliable. Sometimes it doesn't find anything, but when I try again sometime later it works and I can subscribe too.
That's just how Lemmy works, if you're the first person in your server to search for a community Lemmy will have to fetch it, which can take some time.