Okay, just to cover all the bases. Which background atmosphere are you using? Realistic sky, regular sky, none? If you are using a sky background then uncheck the "ground" box. Also try setting your infinite plane slightly above zero in the z height dimension. Which shader are you applying? If it is purely reflective like a mirror or water and there is no sky, then there will be nothing to show up in the reflection. There are no bugs in this situation, it is just something that you don't realize is happening.