Can not get width tool to work

I have tried and tried AND tried to get the width tool to work and so far it only works whenever it seems to want to. 99 % of the time I can't get it to work at all. I've tried starting with the path selected with the black arrow tool then hitting the "W" key and trying to get the width handles on the path but all I can get is the little width cursor with a "+" sign but no handles.  And I have tried every conceivable variation possible but mostly I just end up wasting an hour or so fighting with it until it finally accidentally out of the blue starts to work. CAN ANYONE PLEASE tell me what it is I'm missing here. I've watched every Youtube video I can find on the Width tool but still I seem to be missing some key element of this operation.