I don't like the look of the contact patch on the valve stem. Notice that it is rectangular and not circular. That valve is not rotating. Possible causes:
- Tight guide or seal as suggested.
- Springs setting in the pockets in the head in a way that keep them from ever rotating.
Are the springs tight in the pockets perhaps? Are there any smooth shims under the springs to allow them to move freely or are the springs down against a rough pocket floor that is stopping rotation?