Big plot hole? Spoilers for Fate of the Jedi.

I’m finishing up the FotJ (never read them before) and I’m finally on Apocalypse. Thoughout the book, they keep going on and on about how the Jedi can’t all out assault the Temple (where the Sith are holed up) because they can’t turn off the shield generator protecting said Temple. My question is: where the hell was this shield generator when the Mandalorians assaulted the temple earlier in the series? It would seem to me that if they could just seal up the temple and turn on the shield generator, they could give a middle finger to Daala’s demands. Did I miss something? Or are the Sith better at defending the temple than the Jedi who it belongs to?