Because bad game code some players experienced problem where they can't link main facebook account with a game. Here you can read guide that probably can help you. (not work for some accounts)
Seven Knights Unlink Facebook Account
2/12/2016 - This "hack" not work anymore. Developers fixed it.

1. Link your game account to Google Play/Net Marble. Make sure it says Connected in game settings. THIS IS VERY IMPORTANT, BECAUSE IF ANYTHING GOES WRONG, YOU CAN STILL RECOVER YOUR ACCOUNT THROUGH GOOGLE PLAY/NET MARBLE.
2. Unlink your Facebook account in game. Once Facebook icon grays out, you are done with this step.
3. Close game completely.
4. Remove Seven Knight Apps from your old Facebook account under the Apps setting.
Seven Knights Global Unlink Facebook Account
5. Logout from your old Facebook account on your cell phone’s Facebook App.
6. Setup a new Facebook account.
7. Login to your new Facebook account on your cell phone’s Facebook App.
8. Start the game and go to settings, connect Facebook account.
9. When a new window pops up, asking you if you want to start the game with a new account
Seven Knights Asia Unlink Facebook Account
do not press yes or no, simply force close the game.
10. Start the game again and your game account should now be tied to your new Facebook account.

Thanks to Sam's Knights for this guide.