Why aren't my cores more busy?
Each alternate line is a hyper threaded core, not a real core - count the lines, there are 24, not 12.

If you're doing something processor-intensive, it makes more sense to use real cores, so tasks first share out to all the top line of each virtual pair.

Edit for new image
Image showing  


to the left - 2 simultaneous [yet separate apps] 3rd party app video conversions, still only the real cores needed - presumably the app threads according to the number of actual cores, for speed. If you add more tasks, they will start to add to the 2nd virtual core, but HT isn't as fast as 'real' for intensive tasks.  
to the right -  a single conversion from Final Cut Pro, showing much better 'spread' of resources.


I'm sure someone will come along with a better technical explanation, but in short - no, it's not broken, both CPUs are functioning normally, but maybe the software isn't fully using all the resources.

I have no explanation for why FCP would work differently to 'regular' conversion apps, but the picture shows the difference.