How many Landmarks for the achievement.

[Klungel]

I did the construction of all 12 landmarks, but no achievement.
What i did.: for the airport to build i created on a large flat area a huge materials yard.
With a loaded yard and lots of money, i saved the game.
Then did the construction of the airport and got the airport achievement.
I reloaded the save and started the construction of the Landmarks.
My in-game Landmark-menu shows 12 landmarks.
I'd built 6 and saved the game, then sold them and built the other 6.
No achievement unlocked , not even after waiting a game-year.
So thinking i must be the owner of all of them, i loaded the save with the 6 landmarks and focused on making money to build the other 6.

So , i got now 12 Landmarks - side by side - i own all of them = no achiev.

I did the construction of all other types of buildings except the Ski-resort, the game won't let me.
So , who can tell me what i'm doing wrong or what i need to do to get the Landmark achievement.
Thanks in advance.