A detail is ONCE created; see it as an picture you take. No change afterwards will be seen in this detail (dynamically).
With this said, you have to create your label, make all your settings right and then CREATE. After that you might hide the label… If something changes, you are able to select the detail and choose "Update" to create the detail new...
I make new view in Composer to create the Detail from. Put the labels on in this view and save/update it. Then make the Detail View from it.
Switch to your main view where you want to see the Detail View. Turn on the Detail so it shows, then make the leader, change the properties, etc. (as Heiko mentioned, this will not dynamically update so you will have to repeat the process if you need to make changes)