Found it. OFTEN the canons stop fire .
MEANS - they are ON , it shows shooting.
BUT 1 day no change in target.
AFTER stop shooting of canons. And restart.
next day a spy report shows normal shooted values.
So the Gravity guns "pause" fire - even they are shown as fireing.
NO idea where it does come from.
Because of this I need to spy all targets every day.
And controll if the target still gets damaged.
This way I lost already weeks shooting time. THANKS