If you spot a proof that should be part of a collection you can move it into the collection.
To move a proof into a collection
Make sure your current collections list is expanded first (click the arrow next to collections).
Make sure the proof tile is on your dashboard (look into your inbox, sent or use the search).
Hover over a proof tile on your dashboard (the cursor will change) and drag and drop the proof tile onto the collection name.
Note: You must own the proof and the collection folder to be able to do the drag and drop. If you put the proof into the wrong collection, you can remove the proof from the collection – read how to do this in this article.