This support can't be socketed into this skill - Warning Error Bug?

Hello,

This is just an example i have a lot of those:

The skill unearth deals physical damage. Support skill heft increases physical damage dealt.

Why can i not socket the support skill gem?

It shows +5 Str (0)

But my capacity shows Str (4/9)

It just doesnt give a reason other than

"This support gem cant be socketed into this skill"

It is even more wiered when I raise my dex that had (3/3) to (3/4) i somehow now can socket the gem, but then i click around in the gem menu (clicking on other gems to see the recommended support skills) and come back and now it shows cannot socket this support gem again....