Increasing FEC does not seem to make a difference when I use bounce flash

Some of my photos looked underexposed when I used bounce flash. So I tried FEC from +1/3 to +2 and it did not seem to make a difference in exposure.

However when I started decreasing FEC from -1/3 to -1, I can see a difference in my photos becoming more underexposed.

So what gives?

Suppose the maximum amount of light your flash can put out is 100%. So you ask it to put out 150% by increasing FEC and guess what? It can't do it because it's already maxed out. Now you ask it to put out 50% by using a -FEC and it is happy to comply.

Are you sure the flash is not already at its maximum output? If so, it's not really possible to increase the output. Try increasing your ISO and/or increasing your aperatre and try again.

I guess you guys make sense

I just increased the ISO and it seems to make my photos look correctly exposed

doesnt the 580EX use full power when it is pointed up or sideways on full auto, rather than using calculated ETTL settings?

No. But the power required to properly illuminate the target is higher when bouncing, so it fires at a higher output.

The beauty of ETTL is that it works no matter where the flash head is pointed since it's the camera directing the flash.

It does only work to the maximum power available on the unit - which I have read somewhere might be more than you get at manual 1/1.