Please make sure that you are connected in-game to the account you purchased the DLC with and check that your internet connexion is steady.
 
Then please click on the Restore button in the Options menu (scroll until you are reaching the game's credits, the restore button is just above), it should restore all purchases made with your account.
 
If they are locked only when you are offline, please update your game at least at the 1.7.5 update