IK Rigging of Sandal to Foot

I am using the new Manuel Bastioni IK  character and parented sandals to the feet. If I move the foot the sandal tracks ok...but If I raise the knee or leg the sandal stays put and no movement except a stretching strap... not seeing why the IK rigging is not working as it seem correct. 

Any ideas what I may be missing and nee to look for?

thanks