In that case, you need to upgrade your Water Tank, as the firefighters are overflowed with the fires! Try to improve also the water capacity of your fire engines in the UPGRADES section. If you do this, your teams will be able to extinguish the same fire all at once, and they won’t need to come back to the Water Tank to refill the tanker truck. 

Please take in mind that, for example, if the fire is level 3 but your Water Tank is level 2, the fire engine will need to come back to refill the water at least one more time.