Adding a Sound to My Custom Soundset

So I’ve created a custom Soundset. Then I go to the search panel and find a sound i want to add to my Soundset. This sound has a ‘+’ beside it so i assume it is available for me to add. i click the + and the wheel spins like something is happening but i don’t see the sound anywhere in my custom Soundset. yes, i am in my custom Soundset when I try to do this. Yes I do have a subscription. Where is the sound going? How do I properly incorporate it into my Soundset?

Thanks to anyone who can help