Many cameras require that their focus input is asserted before they will react to a shutter signal. This depends on make, model and camera settings. If possible, it is advised to set the camera to full manual exposure mode and disable autofocus to achieve the most consistent timings.
Different cameras respond at different rates to the combination of asserting the focus line followed by the shutter line. Therefore, it is sometimes required to insert a delay between the focus command and shutter command.
Many common camera model performance values can be found at http://www.imaging-resource.com/cameras/reviews/. While these values may not be definitive for every use case, they are a good starting point.
While the focus line is asserted, most cameras will not respond to communication via USB or physical button presses on the camera. Therefore, it is critical that the focus line is de-asserted using "F0" before attempting to download images or change settings. Some camera models require that the focus line is de-activated between each shot when using mirror-lockup or bulb mode.