Lack of brakes
Hi OldWhizzer, you may want to pull your front wheel, and look to see if you are getting engagement on the whole shoe front and rear, if not take a dremel or equivelent and lightly sand off the glaze, repeat until you are getting about 3/4 contact patch, it will amaze you how much this helps (called "arc grinding" when done on a machine).
IF you already have about 3/4, and cable is adjusted correctly, then the brake is inadequate, or the leverage on the handle is not right.
Mike