Able to add communities from other instances, but not all?

So I want to add the extreme metal community which I believe is on feddit, and the hardcorepunk community on lemmyworld and for some reason they don't come up at all on jerboa or my mobile browser...is this a glitch situation, or am I not adding them correctly? It seems on both jerboa and the browser, adding the ! Links don't pull up communities, they only come up if I type the beginning of the community names in the search bar. The majority come up when I do this, but some such as the ones I mentioned don't come up no matter how I try to subscribe to them. What am I doing wrong?