Yes, Mirror Lockup is one possibility.
Also, if you have Long Exposure Noise Reduction enabled and are taking shots of 1 second or longer, the camera has to take two shots. First, it takes a "regular" shot, then a second,"blank" shot is made with the shutter closed. This second shot is then used to identify noise, which is subtracted from the first image. The second shot is the same length of time as the first one... so if you were making a 4 second exposure, all together it will take 8 seconds to make both shots when LENR is enabled. If you cancel the second shot before it completes (by turing off the camera or removing the battery), neither image is saved... so it seem like no shot was taken.
Another setup that makes you press the shutter release repeatedly is bracketing.... There's exposure bracketing and white balance bracketing. Exposure bracketing allows you to set a range of exposures which will be taken, probably 3 total on your camera, and I think you have to press the shutter release three times for them all to be taken.
Sometimes the shutter release button gets "gummed up" with finger oils, dust and grime, with use and over time. That can cause delays and problems when trying to take a shot. There are several places online that show how to clean it yourself (Google it... but be careful if you decide to try to do it), or you can send to Canon (they'll replace the button entirely), or have an independent repair shop clean it.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
