I've been having this problem for a while. So I have a rig for a robot I am doing and the arm IKs are working as expected.
But when I move the leg IKs the IK isn't working.
The first half of the image is the original position of the IK target.
But when I rotate slightly a bone in the IK chain it works properly.
Here is the blend file with the rig: https://www.dropbox.com/s/6zrcv0caqeh8pmd/rigg.blend?dl=0
I've been trying to figure out the problem for a while but I can't solve it. I don't even know how to google it, couldn't find anything.
A pole target might solve it but I can't use it since as far as I can see I won't be able to limit the rotation of the IK bones.