Thank you for replying back! ^^
The issue that I appear to have - mark the word "appear" - is that, when I today add "SomeGame" to my collection, and tomorrow I search that game there (using the Ctrl-F search feature of my browser), then I get told that the "SomeGame" isn't there, even though the game's store page is still the same.
So, it seems that either I do something wrong, or my collection is, by default, not shown in its entirety.