Hi BSHOW,
You can press the scroll wheel and then physically move the mouse to either side to enable horizontal scrolling. The scroll wheel can’t be nudged from left or right, nor spin the wheel to navigate horizontally.
If the horizontal scrolling still does not work, we suggest that you run the Surface Diagnostic Toolkit. The toolkit runs a series of tests to diagnose your Surface device. The diagnosis and repair time averages 15 minutes or less, but can take an hour or longer. To learn more on how to run the toolkit, click here.
Let us know if you need further assistance.