Starting from the 2014 Winter update, the option to upgrade walls with Elixir can be accessed only when upgrading from level 8 to 9 and upwards. To make sure there are still plenty of resources to go around for everyone, we've added level 12 Gold Mines and Elixir Collectors to the game, as well as better access to Dark Elixir. These upgrades are only available when going to Wall level 9 or higher. 

 

In order to upgrade an entire row of walls using Elixir, all pieces in the wall must be at least level 8 or up. If there are any pieces of wall in the row that are at level 7 or lower, the game will only offer to upgrade the row with gold until those lower-level pieces have been upgraded to level 8. 

 

For more information, please read our post on the Clash forums