Infected Roots not working

Discuss Median XL!
EliToru
Skeleton
2 | 0
I am new to this, so I might have started this in the wrong section but please bear with me.

I have put points in Infect Roots, but most of the time the skill is shown as red, as in I cannot use it.
Does anyone know what I am doing wrong or is this a known bug?

Update:
It seems to work up until I use a auto attack or another ability and then it just stays red after that and
cannot use it anymore