From UESP wiki:
For the mannequins, items freshly placed into the mannequins may make that part of the mannequin become invisible. For example, putting a helmet will make the head invisible. It will become visible again if you remove any other part from the mannequin. Players can put the whole set of armor into the mannequin with an extra ring, then exit the activation, enter again, remove the ring from the mannequin. The body parts will be visible again.
Is that your problem? It may help.