Opening too many prizes in a row may cause the cannon to overheat, and it will be giving only gold.

Please, let it cool down for a couple of hours, and it will work properly again, giving you not only gold but goblins, gems, and elixir!

 

Mystery barrels have a random factor in each cycle. If you get lucky and receive the higher rewards first, the next few deliveries may be lower. It will start increasing again after a while once the cycle resets.