This concern is not overly common, but we have seen a handful of support tickets referring to similar problems.
have consulted directly with Canon/Nikon in the past to determine a
solution. What is believed to be happening are the pins that line up in
the hot-shoe are not making full contact and consequently taking your
flash and camera from ETTL/ITTL to regular TTL. There are 5 points of
contact that need to be firmly seated, and even if one 1 point slightly
off, it changes the exposure on your flash. When shooting horizontally,
gravity keeps the flash properly connected, but when you switch to
vertical-gravity is working against you.
way to determine this is to look at the back of your flash on the LED
where it indicates the ETTL/ITTL metering. If, when you switch to
vertical, it blinks or changes to TTL then you have a faulty connection.
We suggest removing the flash and remounting it. Make sure it is firmly
tightened to secure that all 5 points are connected.
may also want to examine the contacts on the camera to see if any dust
or grease is blocking the connection. Consult your flash owners manual
for recommended cleaning.