Omni bounce doesn't really diffuse the light as much as you'd like to believe. It's a major cause of your problem. Model shooting, I presume outdoors. Omnibounce behaves mostly just like a normal direct flash. too small a light source.
to resolve the issue can try use a larger light source by
1) using a bounce card
2) bounce flash off some wall/directed bounce card WITHOUT your omnibounce
3) use a softbox + studio light