Yeah. The sheets are probably more just blocking the light rather than diffusing it. In the reflections on the glass, you can see how the light still forms a bright spot. When it's diffused properly, that spot wouldn't be there.
Do you have a diffuser for your flashes? That might also help. You could also try dialing back the intensity of the flashes.
Or you could try using a desk lamp with a tiltable neck and shade as an always on light source. You might get more even light from that instead of the flash.