I’ve had similar issues, mostly with recently created communities on other instances and I couldn’t quite pin down the conditions causing it. You can include the @instanceurl suffix in the search and sometimes it helps, but the workaround that works best is to access the community you want directly manipulating the URL, so in your example http://lemmy.ml/c/[email protected]

My workflow has been:

• search browse.feddit.de to discover the most popular communities I’d like to join
• type their URL in lemmy.world and subscribe