Board Thread:General Discussion/@comment-36493046-20190624194222/@comment-38946466-20190624222602

The way things work for me now (I think the change happened with the previous event or the one before that) is that if the collection is already assembled, once I collect all the items, the game will figure it out and will not ask me to complete it. It will just finish the quest.

You really need to search for each item - or hope they drop during visiting (happened to me today) or friend sends them to you (if it is a giftable collection). You can use talismans to search for items to speed things up.