Thanks for the help , but should I always delete it from the collection while keeping it in the subcollection {here I am using Collection name as label only}
Yes, for this case, remove the items from the parent collection (right click -> Remove Item from Collection...), because the items have been copied not moved into the subcollection. From now on, use either Cmd (Mac) or Shift (Windows/Linux) + drag, not drag alone. Also, you might want to untoggle “Show Items from Subcollections” in the View menu.
https://s3.amazonaws.com/zotero.org/images/forums/u383750/xel1q0ks2qm12zzhupuo.png
AND THE SAME ARTICLE HERE IN SUBCOLLECTION :
https://s3.amazonaws.com/zotero.org/images/forums/u383750/d57vdwu78toj3xd3um0i.png
Also, you might want to untoggle “Show Items from Subcollections” in the View menu.