if you've got IK handles going they seem to be doing the job correctly, unless the skeleton is bending in the wrong direction or something... when the IK manipulator is pulled further than the limb it points in the correct direction but straightens out. if the IK manipulator is moved in to a position shorter than the limb it will bend at whatever joint it has... so it looks as if it's performing correctly.

maybe you could enlarge the two problem shots, circle the errors, and give a description as to what it's doing and then what you'd like it to do instead?