I suspect this may be related to auto-focus (AF).
By default the camera will not take a photo unless it is able to confirm that it has focused (this behavior is known as "focus priority"). The behavior can be overriden (in a couple of ways).
In order to lock focus ... the camera will need enough light and a subject with some contrast.
If you test the camera outside on a bright sunny day ... it may have no difficulty quickly locking focus. Do the same thing in poor lighting (e.g. inside with poor lighting) and it may struggle to lock focus. Sometimes you'll hear the focus motor trying to find focus ... and then it gives up.
There could be other reasons for it's failure to let you take a photo ... this is just one possible reason.
There is a switch on the side of the lens barrel labeled 'AF/MF' (auto-focus vs. manual focus) and if you switch it to manual then it should take a shot the moment you press the shutter button (it wont try to focus if using manual focus mode ... of course it also means you have to manual focus or you'll get blurry images). If the focus mode is changed from the factory default mode of 'One Shot' to instead use 'AI Servo' mode then it will also immediately take photos (but it is highly recommended that when using AI Servo mode, you first half-press the shutter ... wait for the camera to focus ... then fully press the shutter when you are ready to capture the shot. AI Servo will continuously keep focusing as long as you keep the shutter button half-pressed.
Note that if the camera is in full 'Auto' mode, many settings are locked out.
The camera also has Program mode (P). P is nearly identical to full Auto mode ... *except* you can change settings and override camera defaults.
You'll benefit enormously from learning the ins and outs of your camera. There are some books that are great for beginners. Bryan Peterson's book 'Understanding Exposure' is very good for beginners. Scott Kelby also has a 'Digital Photography' series of books. These books are not Canon model-specific ... they're generic books that would apply to most any Digital SLR camera user.
Tim Campbell
5D III, 5D IV, 60Da
