Canon Speedlite 600EX II-RT Manual (en)

Troubleshooting Guide
If the main subject looks very dark or very bright, set flash exposure 
compensation (p.28).
If there is a highly reflective object in the picture, use FE lock (p.30).
With high-speed sync, the faster the shutter speed, the lower the 
guide number becomes. Move closer to the subject (p.31).
Move at least 0.5 m/1.6 ft. away from the subject.
When shooting within 1 m/3.3 ft. of the subject, set the bounce angle 
down by 7° (p.34).
Remove the lens hood if attached.
Set the flash coverage to <L> (automatic) (p.38).
When manually setting the flash coverage, set a flash coverage 
wider than the shooting angle of view (p.38).
Make sure C.Fn-21-1 is not set (p.117).
When the shooting mode is set to the <f> aperture-priority AE 
mode and the scene is dark, slow sync is enabled automatically (the 
shutter speed becomes slower). Use a tripod, or set the shooting 
mode to the <d> program AE or fully automatic mode (p.25). Note 
that you can also set the sync speed in [Flash sync. speed in Av 
] (p.54).
Pictures are underexposed or overexposed.
The bottom of the picture looks dark.
The picture periphery looks dark.
The picture is very blurred.