Yesterday I faced with a situation where I saw very often the message of "flash busy" with no chance to shoot. As I understand it can happen when battery power to flash is low, but I use only built-in flash (default kit flash). Please explain when does it happen and why?
The "flash busy" message is shown when the flash is charging and not yet ready to be used.
The flash works by slowly charging a capacitor from the batteries and then discharging all that power in a very short time.
If your batteries are low on power or you have low quality batteries it will take longer to charge the flash and so you'll see a lot of "flash busy", also, if it's dark and you need to use the flash on high power it will take longer to charge.