Cannot be used when leader is in a Mog House.
Cannot be used when destination is in a "dungeon" type area; i.e. Ifrit's Cauldron, Ordelle's Cave, Davoi, Monastic Cavern, Beadeuax, any TaoU 'Remnants' area, etc...
Cannot be used when destination is in an 'instanced' location; i.e. BCNM, Assault, Delve, Legion, Diorama, cutscene, using Synthesis Focuser II, etc...
Try reading here: https://www.bg-wiki.com/bg/Nexus_Cape