Can you post some examples of the problem, and is there also an overall shot which shows the setting so we can better understand the conditions?
The way the Stoffen ought to work (principle, not marketing hype of the company) is that lots of light goes out in a 3/4 sphere, some bouncing off ceiling and walls, some going forward to the subject directly. As the bounce loses power (not 100% efficient) and has a bit farther to travel (by about 1.4x) the forward component should fill in the neck shadows and eye sockets nicely.
Bounce cards work on the same principle...the upward component and the forward component. The result with the bounce card might (or not) alter the distribution of light slightly, compared to the Stoffen. Lumiquest Omnibounce is designed to let the photographer control the relative distribution, rather than it being a fixed ratio.
With a dark ceiling and no walls, any light sent upward is going to be wasted, and this is true of any modifier that permits light to go upward or out to the sides --which is why I use a small sofbox when no ceiling/no walls.