lighting issue

[nelly75]

Hey, I have been doing some basic product photography for a little while but with continuous lighting, so recently I decided to buy some strobes and get in to it more seriously, however...

I have this really odd thing going off, when I take a shot in landscape my gradient background looks nice and even but when I rotate the camera to take a portrait version, my gradient appears to be brighter on one edge! If I rotate the camera portrait left or portrait right the area of brightness from left to right also changes.

Unfortunately I am not using prime lenses at the moment but will change that shortly, anyway when I take the shot at its widest field of view, say 24mm, the gradient is consistent however I rotate the camera, the problem really starts to notice as you zoom in to the product.

Does anyone have a clue what might be happening?

I have attached an image so you can see what I mean... I hope