They are finicky. It's like when your phone won't charge so you have to jimmy the cable to get it to charge... I found a way to make it launch. Always. Select the silo. Click the "attack" icon on the right command bar. Click the ground where you want them to hit, but NOT on a unit. The nuke will then launch. I don't know if this is an exploit, because the planet where I launched them was NOT green (the shade that shows nuke coverage.) Thoughts?
probably not a mobile unit, but maybe a stationary unit, like a tower? the nuke would have to continually change course for a mobile unit
I haven't had any issues with launchign nukes. It can be a bit tricky to know if they are ready/launched on IP distances, but so far they always worked.
I think that the targeting-area is buggy, has happened to me as well. It is not an exploit to attack something that is not shaded green and not able to be targeted, it is just buggy.