The stretcher unit doesn't move when the stretch adjustment handle is rotated.

Sometimes foreign objects can fall between the openings of the stretcher unit. This can cause the stretcher unit to be jammed and unresponsive when the stretch adjustment handle is rotated. 

Unscrew the screw (circled in red) on both sides of the instrument and take the lid off.

Check that there are no foreign objects that could have fallen through the stretcher unit (circled red). Check also that the metal parts are not rusty. 

If unsure, take a picture of the instrument and contact Izon Support for further advice.