It took Sue a long time to master her powers, mostly because of her crippling idiocy. She would try to vanish while people already had a hold of her, and at least once a fight she was discovered by a clever criminal who figured that Invisible Girl must be located somewhere near a non-invisible object she was carrying. To make matters worse, her helpless stupidity combined with her inability to block your view of the TV was extremely appealing to supervillains. They almost universally switched their plans from 'world domination' to 'wedding then rape' when they met her.
"Poor Sarah, it would seem that whenever there is an incline to fall down, she is earmarked for the privilege. At least this one looks steeper and more dangerous than the little hillock in The Five Doctors even if the actual falls looks just as daft."