Lemmy right now is quite buggy when it comes to searching communities from other instances. There is a ugly workaround which works most of the time:

What I do is:

• I take the name “!selfhosted@lemmy.world”
• copy and paste it into the search in my instance
• press “search”
• it shows “No results.”
• I go to Communities on my instance
• I click on “All”
• At the bottom of the list I can see “!selfhosted@lemmy.world” and the Subscribe button

I guess it is some kind of a bug. If I post a url of a post, it also takes qute some time, several seconds, until it shows up.

Other people have similar workarounds: htps://jemmy.jeena.net/comment/7062

Then there is often the problem with subscribing, when an instance is very bussy you might get a orange buttor “Subscription Pending”. The only way of getting subscribed to that community is to click it again so it disappears and trying to subscribe again. For /kbin instances I was not able to subscribe to any of their communities yet.