The final missing item in a collection is typically the rarest one with the lowest drop rate, and it is statistically normal not to receive it. This applies even if you've reached maximum brand loyalty, and maximized appeal on the rest of the items in that collection.
Some items in the special Cast Trip shops may also be obtainable by purchase only. If you're not sure whether an outfit is obtainable by purchase only, you can reach out to Support for confirmation or head over to our Discord server to check about its availability.