Charge tower ult reset

You can't charge your ult immediately after you used it. You should wait until all of the effects of your ult will pass (in your case all of the Bangalore's bombs must explode). You can see when it happens by paying attention on a ult icon at the bottom of your screen. When it stops glowing, turns grey and starts to gain a charge again - then you can you use a Charge Tower. I believe this restriction was make intentionally to avoid abusing of the Charge Towers. Just imagine Gibby throws his bombardment, charge it, throws again and so on. It will be ridiculous 🙂