First of all, I am talking about this plugin: https://wordpress.org/plugins/json-api/
So now regarding my problem: The plugin is working fine and without any problems. All attachments are showen for all posts, but my problem begins, when i want to get an attachment for post and the attachment was used earlier in another post. In this case it returns null (no attachement, or not the reused one).
So to be clear:
Post A Image A --> shows Post B Image A --> doesn't shows
So why this happens, and how to fix this? Thanks for any help!