there was an issue we had where certain objects would not render at all and would in effect be invisible when rendered but perfectly visible within the scene file. after some sifting through attributes keith and fahran were able to discover that the render stats of some objects had been disabled. it started happening with the kids button ties and then happened more notably with the entire eel. it turns out that when you use the create deforemer- wrap tool it messes up the render stats of that object and you must manually reset them. it is vital not to forget to do this before batch rendering otherwise it can be very disappointing.
in this image it shows the location of the render stats found in the attribute editor on the first tab of the selected object. all of these, apart from opposite, should be switched on to obtain a visible render.