What version of Spine are you using to export your data? Are you using the latest runtimes? Does it display correctly in the Skeleton Viewer?
The IK in spine-c (and therefore spine-sfml) works with exports from the latest editor.
If you are still having trouble, please send the .spine file to contact@esotericsoftware.com along with a link to this thread and any notes we might need, like which bones look wrong.