The unique belt “Thorn Slinger” is not properly adding +1 to physical skills. Once a single point is added to a specced physical skill, the +1 levels are not allowed to be added to other slotted applicable skills.
I am having the same issue. However, there is more to it.
Equipping the belt visually shows that all physical skills have +1 point available, when ONE point in any skill is consumed, it removes all the other points available. Going to the other skills tells me I have No points available.
The other part of the problem…by equipping the belt, I am given a point (based on the above issue), if I unequip the belt, and re equip the belt after closing inventory screen, I am alowed to put yet another point in the skill.
If I had the patience to do this process over and over again…one point at a time, I could essentially completely fill all my physical skill points.
The points I add this way remain even after unequipping the belt.