You have to be infested with them or something. If there are like 4 weeds, then he won't do it. But if there are like 40 weeds, ER, YEAH..
My guess at least..
Have you changed your ordinance lately? The beautiful ordinance prevents weeds from growing out of control, but I'm not sure if takes care of the ones prior to swapping. I think having that ordinance in effect might prevent Leif from doing that job if so.
You could always hire people in the forums to pull your weeds.