Wide angle-What's the trick?

So I was shooting a large group family shot this weekend at a wedding. The shots came out ok, but I wasn't thrilled with the lighting.

I used a 17-40 on a 30D body. I have a 430ex flash, but was actually using a 550 with a homemade bounce card. And it was like the left side of the group was a tad darker.

I've run into this before, using my 430...with and without a defuser, a flash bracket, etc.

Seems like they would design a flash unit that had a bit of a contour to the head that would spread the light out some, ya think? What's the secret to getting good lighting on a large group with just a flash unit?

I suspect the answer is just more powerful flash unit.
May be your bouncing card wasn't even enough...

Pull down the wide angle diffuser.

You could perhaps use the 550 as Master and the 430 as slave - in Manual or Ratios mode. The slave can be on a light stand or tripod.

Was the groups evenly arranged relative distance to your camera? Light falloff may be at fault here too.

There are a number of possibilities. Posting a sample shot with EXIF info would help greatly. Otherwise, all you will get is speculation.

The reality of shooting groups with a wide angle lens, if your subjects are lined up in a straight line, is that the people near the ends are significantly further away from the camera than the people in the center. Depending on how you're bouncing the flash and the diffusion devices you're using, the inverse square law can sometimes reveal itself to make the subject near the ends darker.