Summary of possible causes:

Poor dose selection
Drug identification errors
Loss of injectate
Inactive local anaesthetic solution
Misplaced injection
Inadequate intrathecal spread
Anatomical abnormality
Failure of subsequent management (e.g. testing block)

My take home messages from the discussion were:
1. CSE is a good option if time permits
2. GA is an acceptable second line alternative
3. Shake heavy bupivacaine ampules before use