I just got the 468ii a few days ago. Which model do you have?

It seems like in TTL mode the flash should pretty much adjust its power for good exposure. With mine, I've tried just about every shutter, aperture, ISO setting and the exposures are coming out nearly the same, all well exposed.

I too have a lot to learn about using flash and first thing for me is to figure out what the flash can do and how to make the flash do what it can.

It must be the 468...

http://speedlights.net/2010/10/10/yongnuo-yn-468-flash-review/

Can you post a sample pic of the underexposed image and settings?

You can use the camera to increase the flash exposure...I'm finding 0 to -1 suitable to my tastes, although I keep referring to Skippy's thread on pushing the exposure to +1, so it is still a toss-up for me. I need to spend the time on that subject, but my knee-jerk reaction is to avoid blow-outs and post the shadows...

I gave up trying to figure it out for now.  Been using my ring flash when I need a flash!  LOL

I'm starting to think this flash misses sync enough to cause issues, when in TTL mode - and that is on camera... I know the optical slave modes are flakey, the wireless triggers seemed to work fine at the time, but now I am reconsidering as there were a few miss-syncs...

The bottom line is that I am not getting consistent results with this flash...

Maybe I am just confused, but I am now once again having problems getting this to fire while detached from the camera. I can push the test button on the trigger attached to the camera and get the flash. I was also able to get it to fire with the 50mm on the camera and the flash set to 50mm, but then I changed the f stop and I haven't been able to get it to flash since while triggering the camera.

Now, I am very new to all of this flash stuff, so maybe there is something that I am just missing. Unfortunately, the manuals for both the triggers (rf-602 receiver and rf-600 transmitter) and flash, are nearly worthless in terms of explaining anything.

Back to the internet to scour for information.

Edit!

So, it turns out that the flash doesn't want to work when using the LCD on the camera (while the flash is off camera - I forgot to check it out while it was ON camera as my batteries were flashing red so I tossed them on the charger), but seems to fire just fine when using the viewfinder. I am feeling much better right now. I am doing a lot of learning still, but I am writing down notes and keeping them on the backside of the flash for future reference until I get a much stronger grip on how to use this thing.

Manual mode so far has been steady. You have to be in manual to use the wireless triggers and my wireless triggers have not missed once yet (same RF600/602) - except they do not trigger in Live View.

Page 207 of the manual:
Cautions for Live View shooting
A non-Canon flash will not fire during Live View shooting.

I hadn't read that manual when it comes to the flash.... With that said, I guess my confusion came when it DID actually fire a few different times when using Live View. No idea why it did, since it isn't supposed to, but it did.