I added my entire collection sometime ago, but I've been having some trouble adding specific games to it. This isn't just the collection system either, but also the search bar. It only displays so many results with no option to see more, so if you add a game from a large series all the other games may take up the list without you finding the one you're looking for.
For example, this happened when I tried to add Resident Evil 0.
Hmm... Resident Evil 0 shows up in the quick search bar for me, but if you ever need more results go to Browse Games and use the Search Function at the bottom of the screen. This needs to be made more obvious, but it displays more results.