Canon Speedlite 600EX II-RT Manual (en)

Troubleshooting Guide
Radio Transmission Wireless Flash Photography
Set the master to <'M> and the slave to <'x> 
Set the transmission channels and wireless radio IDs of the master 
unit and slave unit to the same numbers (p.63-65).
Make sure the slave unit is within the transmission range of the 
master unit (p.58).
Run the transmission channel scan and set the channel with the 
best reception signal (p.65).
Position the slave unit at a location with possible clearest view of the 
master unit.
Face the front side of the slave’s main body toward the master unit.
The camera’s built-in flash cannot be used as the master unit in 
radio transmission wireless shooting.
When using autoflash photography with three firing groups A, B, and 
C, do not fire with firing group C pointing toward the main subject 
When shooting in a different flash mode set for each firing group, do 
not fire with multiple firing groups set to <a> or </> 
pointing toward the main subject (p.81).
Set the shutter speed 1 stop slower than the maximum flash sync 
speed (p.61).
The slave unit does not fire or unexpectedly fires at 
full output.
Pictures are overexposed.
<k> is displayed.