30D at times wont fire a shot when pressed...foucusing issue?

My 30D has started to "lock up" and won't take a shot although it has a focus lock on the subject. Typically seen at at shutter speeds below 1/100 in both M and Av modes. The circle in the right corner of the viewfinder will flash on and off...

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What focus mode? Does it lockup when using AI Servo?

ETA: If the circle is flashing then it is unable to acquire focus. You'll need to either use AI servo or manually focus. Turning the focus wheel is not enough, you must switch the lens' AF off.

If your lens is on AF and your camera on One Shot, your camera will not shoot until it achieves focus, unless you use back button focusing and shutter button only for shutter, in which case you can just let go and it will shoot. If you camera is on AI Servo and the lens on AF, the camera *may* not fire until it has something resembling focus. Happens a lot in sports where high speed will splurt rather than machinegun. If the lens is on MF, shutter button is broken and you need to send in the camera to fix it.

I'm guessing it is a dirty shutter button or the sensors in the shutter button going bad. The same thing happened to my 30D. I sent it to Canon and they fixed it for free out of warranty. Of course, they didn't really explain what they did to fix it, but the problem is now gone.
I've also heard reports of people fixing this by spraying just a bit of electronics cleaner into the shutter button (holding camera upside down to avoid cleaner getting into too much of the cam). I went the Canon route myself though.

Not that uncommon, see:
http://www.flickr.com …iscuss/72157600​619735796/