A: Could be one or a combination of reasons.
- Raytracing isn't turned on in the render settings
- Even if its turned on, the refraction traces may be too low to pass through the surfaces [also, max trace settings must be high enough to cover both reflection and refraction traces].
- If Final Gathering is enabled the tracing should be checked as well, its very much like the raytrace settings.
- Last but not least, the material's refraction tracing. Refraction Limit on standard Maya shaders and Max Trace Depth on mia_materials.