Model disappears when rendering in iRay

Hello

In a few of my projects my model disappears when I try to render in iRay. I have checked over the settings and they are exactly the same as another similar project I have which renders fine. I've tried moving the camera out a bit, making sure alpha is not turned on and a few other changes but none make a difference, the model is literally invisible! Any suggestions on what might be causing this?

Thanks