On the mission "Airship down" where you take down the Brotherhood at the Airport, i have the objective "destroy the brotherhood generators 0/3"

I've watched some Youtube walkthroughs & folk just shoot them once & then they blow up. However on my game they are indestructible.

I've emptied an entire arsenal into them (inc Mini Nukes) & they remain intact.

Either im missing something obvious or this is a game-breaking glitch

Please help ASAP as i currently cant do anything until i can destroy these generators

Thanks

Anyone? Game is basically unusable as a result of this