Could be that the specular settings need adjusting, could be that the corneas (if there are any) are opaque and hiding the other eye surfaces. Go to the Surface palette, select the Cornea material (if there is one) from the list and see what the Opacity setting is: if it's 100% try setting it to 0%. If that isn't it, and if the irises are visible in preview but not in renders, try selecting the eye materials, go to the Advanced tab of the Surface palette and turn the Specular strngth down and/or the Glossiness up.