Photoshop CS - non-funtioning "Shift" Modifier Key for Paintbrush, pencil, and Eraser.

After modifying the keyboard shortcuts in Photoshop CS, the "Shift" modifier key for the Paintbrush, pencil, and Eraser stops working from point to point. The modifier does continue to function if it held down to make straight vertical or horozontal lines, but stops funtioning for a diagnal from a "shift" click to the last click.