Can't scrub

Newbie question here,


I am running Avid Symphony 6 on a 12 core mac pro running Lion and after spending a couple of hours going through Lynda tutorials, suddenly I can seem to scrub properly in the source or the timeline. When I grab the playhead and move it, I just get a still frame, and it only updates when I let it go. I can hear the audio scrubbing fine, and I can JKL around no problem. Also, double clicking clips doesn't immediately load them into the source window, I have to click on the window for it to update.

Have I just accidentally pressed some obscure "no scrubs" button (like the silent but deadly "W" key in FCP)? Or is this a bug or what?

Thanks in advance,
Chris