It seems that sometimes after deleting a keyframe that had IK applied to it will cause the track slider to become un-draggable, you can still move it by clicking on the track, however you cannot drag it at all.
https://gyazo.com/42347e94afdd4f9e7d32f14b7ce42a86
The only way to make the dragging work again is to close the animation editor and re-open it.
I am having trouble reproducing it 100% of the time, it seems it is not a constant issue however maybe I am missing a step, will update if I get a 100% repro