Why won't Windows use the other CPU cores?

Many apps don't use all cores. They aren't programmed for it. Then they will use one core and will overload it.

But you can tell Windows to run that process on 2,3, ... and in all cores.

For doing it, follow these steps:

1. Open Task Manager
2. Select tab Processes
3. Right click and select 'Set Affinity'
4. Select on which cores you want to run process.