Flash: Fired but Severly Underexposed. Why?

The other evening, I took this photo:

The photo is taken f/5.6 1/80s ISO 800 with the flash. I didn't have an external flash with me and just wanted to add some light, f/2.8 1/40s and ISO 1600 or 3200 would have been good enough.

There are 2 exposures:

• The ambient light, it's enough to give the feeling of the area/background. A lot of white walls for bouncing spilled light…
• The flash exposure for the man with the dog on his lap

The ambient exposure is right but the flash didn't fire. Actually the flash fired, eveybody saw the flash the as you can see it didn't work.

What was the problem:

1. The popup flash on the Canon 7D was set to Wireless Enabled, which is OK
2. The popup flash on the Canon 7D was set to Wireless Master Flash Enable and Do NOT participate in the exposure

This changes the popup flash from being a Master Flash and Do NOT participate in the exposure to fire normally without trying to control another external flash and I got: