I've had this problem too and I searched the web a lot for a solution, to no avail.
I was finally able to fix this. Firstly, it doesn't show as 'Unknown Artist' only on your device. It also show as 'Unknown Artist' on iTunes. If you browse All Aritsts, and scroll all the way to the end, you'll see those tracks listed there as 'Unknown Artist'. That's how iTunes is seeing them, and that's how your device will see them, too, when they are sent to the device.
1) Remove the tracks from iTunes by clicking DEL on them
2) Using a tag editor, remove the tags for Artist and Album Artist, then save.
3) Now fill again the details for Artist and Album Artists on those tags, then save again
4) Add the tracks again to iTunes.
After several different approaches, this one has solved the problem for me.