Why can't I add silver to my cave during a spy action?
It could be that your cave has not reached the maximum level of 10 yet. Only then can you store an unlimited amount of silver. Before level 10, you can add an additional amount of +1000 silver per level.
If you have sent a spy, the spy also carries silver with him. The spy's silver plus the silver in the cave add up, so if they both together reach the maxium amount of silver that can be stored, you cannot add more silver to the cave.
You can find more information about the cave in our Wiki.