When you create a SmartObject from a List or Library, you must select to use in in a List/Library or a Site/Subsite. 

List/Library - Select to allow only this list or library to see and use this specific SmartObject in its workflow.

Site and all of its subsites - Select to allow any list or library in the site, including all of the site subsites, to see and use this specific SmartObject in their workflow.

If you are trying to use a item from a list inside of another list or library, you will need to make sure that your SmartObject is in the scop of Site and all of its subsites.