Help With my ragdoll

im making a ragdoll and everything has been going along smoothly. the rigging and adding the constraints. the only problem I seem to have is when I hit play, some of the vertices on my model wont fall to the ground. mainly the whole head is stuck in the air and also the chest and stomach. the only parts that fall to the ground are the legs and the arms. and they don’t really fall, just sort of stretch to the floor. please if any one could help me with this I would really appreciate it.