trmiv's tip was right on the money, thanks!
Logitech Mouse Wheel Does Not Allow You to Zoom in Game
You may find that scrolling the mouse wheel on a Logitech mouse does not allow you to zoom. To resolve this issue, perform the following steps:
1.	Update the driver for your mouse from 
http://www.logitech.com.
2.	After updating the driver, restart your computer.
3.	Once back in Windows, right click on the Logitech mouse icon in the taskbar and select Properties.
4.	Click the Buttons tab.
5.	Make sure WebWheel is selected in the #2 Option.
6.	Under the Scroller section, select Use MSOffice Compatibility Scroll Only and Scroll in Active Window Only.