You cannot earn Festive Reward Tokens if you are using a ship which no longer has an active Snowflake on it. It might be that you are re-using ships?
I'm not sure if you are limited to an amount of Festive Reward Tokens which corresponds to how many snowflakes are remaining? Please make sure you use your Festive Reward Tokens as they have no value other than removing a Snowflake off a ship.