We have all been in the situation when resources seem scarcer than they ought to, but please keep in mind that they cannot disappear by themselves and the game will never make purchases on the player's behalf. Resources are needed in order to make an upgrade or buy something, so that is the best place to look for an explanation. Perhaps you are upgrading a lock or a trap? If that's not the case, check your Defense Logs, perhaps someone has raided you and stole some of your resources.