Surely you have a steady table or surface somewhere, try clamping it on then try pressing in the release button and lift the back up, if it still doesn't work get your GF to press the button in and you try and lift the back off. I have just tried with my clamp and if I hold the hook which hold the wheel in, I cannot press the button, they are a single piece of plastic so it should be moving when you press it in. I find when they get stuck it is easier to press the release button with both thumbs as it allows you to put more pressure on. As stated above though, don't try too hard as you may end up breaking it.
I don't see though why it would catch on it, I know how they work on the inside (mine's no longer white and grey, lets leave it there).