They were probably set up in such a way as to not allow claiming them. That's a thing with projects made temporarily free. If the creator decided to go that way, your only option is to actually buy them.
Viewing post in Unable to download items from my library
Because there's a difference between downloading and claiming, The latter results in you owning the product as if you had bought it, which gives you access to all future versions. So the creator may not want that. Instead, they can let you download for free during the promotion, but no further. Guess the difference could be more clearly communicated when you go download. See also this recent topic.
Sorry, but that makes no sense at all.
Sure, the distinction makes sense, of not wanting to give someone access to future updates, but conflating versioning with downloading is insane. What if someone did download something, but then gets a new computer and is unable to access stuff they had added to their library in the past?
It's not particularly difficult to implement this correctly; just put a tag on the library item's data saying that it's locked to a specific version, and they can't download any newer version than that. Saying they can't download it at all is completely wrong on just about every level.
You can also very easily keep the game archive you downloaded, since there's usually no DRM to prevent it. And sure, that's not always possible, but for a game you downloaded for free during a promotion? If you lost the download and want it again, no offense, but maybe you should consider buying the game if you like it so much. That way you'll own it in perpetuity and you'll be able to download it again any time you want from any computer.