they have hidden requirements. the hints are
Mamu: Requires a specific tier 3 building in the village, doing something you wouldnt normally do, is vaguely related to cold, youll know immediately when starting a run.
Nessa: Requires a specific egg, is bird related, doing something you wouldnt normally do, is vaguely related to warmth, is not possible in zone 1 but is possible in standard mode.
Let me know if you need more help or the exact solutions