Thanks for sharing the scene, that's helpful. Here's what I did:
Selected all the skeleton objects:
Then I ran:
Ziva Tools > Run Mesh Analysis
If you look in the output in the script editor you can see all the meshes that have errors.
There are several with negative volumes as was suggested earlier
Selecting one at random reveals a negative scale in X
I would encourage you to build the setup again with all the skeleton geometry's transforms frozen.
Hope that helps,