Sometimes the IK solver does not bend a bone chain when the bones are aligned (no rotation angle in the rest pose). The user needs to add a tiny rotation to the bones so that the resolver knows how to bend.
here is a gif that demonstrates what i mean:
However if there is an IK Target defined, then the location of the IK Target bone could be used to remove the ambiguity into which direction the chain shall be bent. Is that a feasible solution ? Are there any drawbacks?