Since your plane doesn't appear to be reflecting light in the same way as your spheres regardless of the reflection formula you use, I would check that your surface normal is pointing the right way for both objects. It's almost certain your sphere surface normals are pointing inwards of the sphere whereas your plane surface normal is pointing upwards (or vice versa).
Remember: the surface normal is typically defined as always pointing "outwards", that is, if V is the direction of the ray as it hits the geometry, then dot(V, N) < 0 (so that dot(R, N) > 0 where R is the reflected ray). You can define it the other way, with it pointing inwards if you want, but you have to be consistent and make sure this is true for all your geometry, since the reflection formula kind of "needs to know" which way to reflect your incident vector, and that depends on the orientation of the normal vector.
It's fairly easy to enforce if you're only doing reflection, since you can do the dot product check and flip your normal as needed, but it gets messy when you start doing refraction/transparency where non-watertight meshes are physically meaningless (they have no boundary) and you can't flip normals since you need to keep track of which object(s) your ray is currently inside of, therefore you have to be a lot more careful in these situations to make sure your geometry is self-consistent. For opaque reflection you can just hack the surface normal to make it point in whichever direction you need and you're good to go (again, because with reflection only your ray is always "outside" and you know it).
EDIT: if you could show your code for the GetSurfaceNormal() method, that should help track down the bug.