Fail to have a complete UV unwrap after sculpt modification

Hi im new with Blender. I have hired a sculptor who finish a sculpt and was given an OBJ file. I try to modify the uncolor OBJ file by sculpting on it using Blender sculpting tools. As I progress my learning and try to UV unwrap the model, Only those areas untouch by my modification sculpting are unwrapped while those areas modified appear as pink. Apparently the unwrap fails as it covers only those areas untouch by my modification as shown in pictures. The vertices also dont appear uniform after my modification. Could someone help to resolve this issue Im having. Your help is appreciated. Thanks.