I have had better luck using a combination of both 2-4D (the old Ortho Weed-B-Gon formulation) and Triclopyr (Ortho Brush-B-Gon). It's a tough weed to successfully get rid of and in my experience it takes several treatments with the 2 herbicides a few weeks apart.
Be careful not to spray this on bushes as the Tricolpyr can cause damage or death to bushes.