Scrolling + Command Key not increasing font size

I'm having a problem. Usually, if you are using a web broswer (ie Safari, Firefox) you can increase or decrease the font size by holding down the command key and scrolling up or down, respectively. This has worked for me in the past, but recently, it seems to ignore increasing font size and it merely scrolls up or down. The command key works just fine. The scroll wheel on my mouse is fine and the two finger scroll works fine. I can work around this by using "command and -" or "command and +"but that can be a hassle sometimes. Does anyone know how I can fix this? Any help is much appreciated.