How do you know it is a fan under the floor boards, that would seem like an unusual set up. If you can see it you should be able to tell if it is working or not. You would hear it if it was working. Could do with more detail really. Are you sure it is an inline fan and not a back draught shutter. If it is a fan and was not working it would reduce the flow but not stop air getting through altogether. How does a fan in the shower get ducted to floor level?