Problem solving help with 3D character model

No actual render work needed, just advice from someone experienced with high level photorealistic character model animation. Character was originally very high poly (+11Millions Polygons) in zbrush and a low poly model (with normal + bump map, diffuse for each element) was made for easier animation use. Screengrabs titled 26 and 28 are the high poly version and the others in low poly (ortho view and renders), then some composited versions (low res). I'm seeing a difference in the mouth and nose as well as detail overall, the chin and neck appear to be a different size/shape. Would be very grateful if someone could chat with me and solve the puzzle as to why they look different? Don't need anyone to work on it, just need suggestions for solutions. Thanks so much