Fixing a stone window sill that's not directing rain away from the brick

We have two stone window sills that weren't installed properly. When it rains, water ends up running underneath the sill and down onto the brick wall. How can I fix it? I don't want to remove and reseat the sill, since I want to keep costs down.

The sill has a drip groove on the bottom, but the tilt on the sill "overpowers" it, so the rain ends up cascading down the brick wall underneath. Argh, help!