this:

http://www.vgchartz.com/games/game.php?id=17561&region=All

i don't know why but there is not the "ADD" button

It says there are zero sales so you must be lying...

Click "Add" next to "Tracked" or "Wishlist", then the Add option appears next to "Owners".

There's no release date so I'm guessing the page is assuming that the game hasn't been relesed yet, a measure to rpevent people from adding games that aren't out yet to their collection.

The 28 people that do have it on their collections most likely added it before VGC 2.0

That's odd...the only thing I could do about this is delete the game and remake it but, there's probably a easier solution...

It says add too me.
Owners: 28 (Add)

That's how it is for me.

I don't know what causes it either, but this usually seems to work for me. I'll always add a game to my wishlist if there's no add button, and then later go to my wishlist. Many times something in the scripting cleans up and the add button appears, which is when I add it and remove it from my wishlist. It's not the only issue that I've seen with the database, though- some games I had to have oliist send me links for since the game would not show up in a search for it. (Yet the database knew it was present by spitting back errors that the game already existed in the database when I tried to add it.)

This newly added feature is introduced not only to prevent people from adding not yet released titles to their collection but to stimulate (i.e. force) our members to help the database getting more complete.

P.S.
gamefaqs.com is the best place to look for release dates etc.

^^ or to use the wishlist feature. games with release dates that aren't released yet you can not add.