symmond balancing spindle stuck

I installed the symmons valve and I noticed only luke warm hot water. A bit of research and I took apart the valve and the balancing spindle is stuck, I guess it should move when shaken. Any thoughts on how to get it unstuck? I tried in vinegar, not sure how are to tap it?

Advice.

Sometimes a valve has a high limit temperature stop that can be adjusted. I would check for that first.