I think you are correct, not assigning an index of refraction to the detector volume is likely the issue. If I remember right when no index of refraction is defined for a volume the photons are killed at the surface and hence never enter the detector volume. A quick solution would be to assign an index of refraction to the volume the detector is assigned to, the value of the index of refraction should be equal to the surrounding media so no reflection or refraction would occur.