Parts of my mesh are hidden when armature modifier is turned on I've been following an online robot modelling tutorial and I'm having an issue where part of my mesh is invisible when the armature modifier is turned on. When the modifier is turned off I can see them again. 

I had already started animating my model before I realised two of the cylinders on his legs were missing! 

I'm new to blender and i'm totally lost as to what the problem might be, any help would be welcome!
You get a twist problem on your "Track To" constraint on KneeCylinder.L / KneeCylinder.R.

(Try set influence of this constraint, and you will see your cylinder).

Quick fix by Rolling your bone by 180° (edit mode, and set roll to current roll + 180°)