It's not clear whether you're missing the outside humidity data, or the image.
Assuming the latter. I'm not familiar with the neowx skin, but looking through its GitHub repository, it looks like the skin.conf file specifies that images be generated for outside humidity for the day, week, month, and year, but neowx doesn't actually use them.
For example, the template index.html.tmpl includes references to daytempdew.png, but none for dayhumidity.png.
You can test this by browsing to dayhumidity.png instead of index.html. You should see a perfectly fine image. It just needs to be incorporated into index.html.tmpl.
OTOH, if you're missing the data, then we would need to see the log.
-tk