balding tail

[Ubu Noir]

My sensitive gerbil, spoo, has a balding tail... i'm assuming she's balding
it herself, though i never see her doing anything.  What could be wrong
with her?  She's got an icky nose which i'm taking care of.  I thought she
wasn't getting enough to play with or a clean wheel often enough, so i've
been paying lots of attention to her lately, and gave her and her cagemate
metal piping and loads of stuff to chew on... yet her tail's getting
worse... any ideas how i can help or stop her?
  THanks, Kelly