Verticals underexposed

Question. Why are my vertical images underexposed? Here are 2 images as examples. Shoot the same subject horizontal and vertical and only the vertical's are underexposed. This was with using the lightsphere II clear with flash aimed at the ceiling in both orientations. I took the lightsphere off and used plain bounce flash and the vertical is still underexposed but not as much. Using a Rebel XT with 380ex II flash and low white ceilings (I live in a trailer).

Any solutions or ideas as to why this is happening and how to stop it is greatly appreciated.

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Perhaps the fong has put too much weight on your flash mount, and the flash is not making proper contact when vertical? Does it wiggle at all?

I'm also thinking it could be an electrical contact issue. I can't think of anything else that would cause this phenomenon. Make sure your camera's hotshoe isn't loose.

I checked them, nothing is loose but very clean. I have another rebel xt so I put the same lens and flash on it and get the same result. So I tried a promaster 7500dx flash on both cameras, same result. I changed zoom lenses, same result..all these with the same settings, only thing changed is orienation..verticles are underexposed.
I even changed scenes..same thing.

I don't know why, but I'd like to help you figure out.
1. Set the camera to f/5.6 and 1/100.
2. Set the flash on manual in the landscape orientation and by trial and error, get the correct exposure.
3. Without changing any thing on either the camera or the flash, change it to vertical and try a frame.

Do you see any variance? You shouldn't. If you do, I'm not sure where to go from there. If both images look identical, then it could be because of the orientation of the flash head itself. Go back to auto and try adjusting the FEC on the vertical orientation until the images look identically exposed.

Bottom line is this experiment will help you understand your equipment better. Maybe a couple of other people could also try this?

Weird problem for sure...

I know you checked the hotshoe but did you check the flash in the hotshoe? If the flash can pull away a bit from the shoe in the vertical it may cause a problem. And it doesn't have to pull away far.

With my LSII I rotate the flash head up when you go to vertical? Is that what you are doing? I don't know what it could cause though.

I did and there is no variance at all when in Manual flash. I just had to go from 5.6 to 8.0 and there is no variance from horizontal to verticle..so where do we go to from here?
I had to do a + 1 1/3 FEC to get the same result in vertical as no adjustment at all in horizontal. There is no way I could do a FEC of that when doing a wedding or reception everytime I needed to go vertical.

It is not the connection.....
I just read the EXIF data and the flash fired for both, so it can't be that it is dis-connecting.

Only thing I would say is.... When you flip it to portrait orientation. do you move the flash head so it is pointed at the ceiling? As Titus said

Yep, it is pointed at the ceiling in both instances..

If you take the fong off. Does it still happen?

No, take the fong off it works fine..even works fine with the flipit or better bounce card..it only does it when a diffuser of anything is over the flash head..even a soft box so it isn't limited to the fong.

I think it's the weight of the Fong or the soft box putting just enough more leverage on the mount that it is pulling one of the various little contacts up just enough that it isn't making a good connection or it's tweaking a bad solder connection or something of that nature. It's hard to imagine that having something over the lens is the real culprit. Have you tried the pull-out wide angle lens over the flash? That wouldn't add any significant weight, but it would cover the front of tube. You might also try a Stofen gizmo and see if that does the same thing. Or tissue paper.

Although the camera data will tell you that the flash fired, it won't tell you much about the quality of the conversation between the camera and the flash.

Ok, I'll try that..even when I use the velcro strip around the flash and use the lumiquest soft box it does it.

Does it happen when rotated 90 deg left as well as 90 deg right?

Yes, any bounce whatsoever with a diffuser on..bounce with nothing on the flash head is fine..I'd hate to have to give up my diffusers. I don't have a stoffen or something lighter to try..if they are all of a sudden too heavy then I'll have to quit but this only started this past week...