How Do You Turn On Pressure Sensitivity On The Wacom Bamboo Tablet?

I have just bought a Wacom Bamboo tablet and when I used it for the first time, the lighter I used the pen, the lighter it showed up on my drawing program, then the harder I pressed the darker the line was. Turning it on today and using it in the exact same program, it doesn't have that and applies the same opacity no matter how much or how little pressure I apply to the pen. How do I turn that back on..?