Can't add symbol to a pubished library - add button is gone

Hi,
I've created a library to a document and published it. I want to use this library in many other docs and add items to it as I go, from many other documents. But the the Add ("+") button is gone in that spesific folder. instead there is a lock icon.
Why can't I add an item to this libray.

Here it is in other libraries where i can add:

Thanks