I'm stumped. Has there been a change to your hardware or software setup? Have you tried shutting down and restarting your computer (in case it's a glitch)? If the answer to these questions is no and yes, respectively, maybe include a screenshot of your app.
I find that selecting the Brush tool and clicking on the Stroke color swatch in the Properties panel allows me to freely sample from the color picker, the canvas, or anywhere on screen. I'm using FW CS6 on Mountain Lion, and have a non-Retina MacBook display.