The above topic states it best... when fighting against the Apocalyptic Beast my Spirit Reaver refused to cast... the attachments I had on were:

Loudspeaker I & II, Mana Tank I & II, Optic Fiber, Tranquilizer, Mana Channeler, Mana Conserver, Condenser, Stoneskin, Ice Maker, Turbo Charger

With this combination equipped, the BLM refused to cast anything...even deactivated and reactivated... still nothing when deployed.

Some have hypothesized that if the enemy has "no" magic weaknesses; then the blm automaton will just "stand in ready" but do nothing...

This has happened... please test and make adjustments...