This weekend I was taking photos at indoor track meet. The lighting was fairly dim, particularly later in the day. Since there was a fair amount of movement (trying to photograph runners) I was shooting at shutter speeds of generally 1/400-1/500. ISO of 1600. At times the shutter would not release. I would try then try to reduce the shutter speed until I could take the picture. Any ideas? Was it simply not enough light for shutter speed I was attempting to use? Anything else I could have done differently?
In low light autofocus might struggle due to low contrast between the subject. The best way round it is to move the focus point to a position where there is more contrast. For example, try shooting a white cloud in daylight. The camera won't be able to lock on due to lack of contrast, but by moving the focus point to the edge of the cloud and the blue sky, it will be able to lock on and take the picture.
You could shoot in manual mode, where you use the focus ring on the lens, to set the focus point and then press the shutter and the picture will be taken. This can be done handheld but using a tripod may be better.