Dear Joanne,
I'm wondering aobut a couple of things with your daughter. The first is whether she could hvae some sensory integration issues. This might mean that the water hitting her skin could be irritating or downright painful for her. How is she about her clothing? Are their certain textures she find uncomfortable? I would check with the physician to find out more about this.
Does she struggle with transition, meaning you need to establish a consistent routine around showering.
Is she depressed or does she have any body image issues? Children (and anyone) who struggles with depression often have issues with hygiene.
I would talk with her physician and attempt to implement a reward system. I would begin by giving her a small but meaningful reward after each shower. Then as she gets more accustomed to the routine, you move to rewarding her every three days to once/weekly to no reward. Rewards can be a new accesory, a favorite treat, stickers, gum...
The other thing I am wondering is if she sees a social worker. If the physician is unable to help you find a solution and the problem continues it would be a good idea to get her some therapy. A social worker specializing in these types of problems would have some good ideas to motivate your daughter and teach ehr self-care skills.
My other suggestion...have you tried telling her that she can sit and cry if she wants but that she cannot come out of the bathroom until she takes her shower? I would only do this after pediatrician has rules out that there are no other issues going on. It may be that she has become dependent on you. Setting a limit that she needs to shower (or bathe) herself might be tortuous for you and her at first but within a few days if she sees you mean business she may comply.
I hope this is of help for you.
J.