I think items such as bldgs and trees should be able to be reused if you already ha e them from a previous challenge
Currently the team challenge is programmed to use only new assets added for the current week. This allows all players an even chance to participate in the competition. We have passed your comment to our product team. We will try to implement other uses for the assets. We appreciate your time and support.
As the team challenges do not start on an even playing field for all participants, as they are biased to those who spend money on extras like energy, then it would make little difference to fairness if those with assets gained in previous challenges could use them again. Perhaps it would be be fairer all round if nobody could buy themselves an advantage!
Customer support service by UserEcho