Clicking USB mouse or pressing USB keyboard while going into standby may prevent mouse/keyboard from waking system
Consider the following scenario. You configure an USB mouse or keyboard to allow it to wake up the system. When the system is entering standby, if you either click the mouse or press the keyboard during this period of time, randomly you may find that you cannot use the mouse or keyboard to wake up the system. If you don’t click the mouse or press the keyboard, the issue won’t happen.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB2541103.