I recently purchased a Speedlite 600EX-RT for use with my 5D Mark III. I'm shooting with a 17-40 f4/L lens. Particularly when using the flash in the standard forward position, I get extremely severe underexposure when shooting in P mode with the flash in either High Speed Sync or First Curtain release with E-TTL2 on in evaluative mode.
There are perhaps one or two fringe areas of the image that are exposed (less than 3% of the total image) and none of them are at all close to the chosen focal point. Many images haven't even had any portion of the image properly exposed. I'm using Center Weighted Average Metering for my metering setting.
Any idea what could be causing the issue? Does the metering setting for the camera and the flash have to match for proper results? Is there simply an issue with the communication between the flash and camera body? I'm really not sure what is going on and why I'm getting such poor results with the flash. I notice it also seem to do much better when using the flash in a bump flash configuration.
Edit: As mentioned below, I do realize there is a flash exposure compensation adjustment in the flash that I can make to boost the exposure, but I'm trying to figure out why it is misbehaving rather than simply trying to work around the problem. I'm specifically looking for any additional insight in to what could be making it underexpose by multiple stops.