Ehm, the tower is only firing when you enter the Radius? 
Have you watched at your BP at Runtime, checking if something closes the gate?
Normaly you should open the gate when he enters and close it when he leaves.
You can also change your setup to something with a bool.
Let the Tick check a bool with a branch. Lets say “bSomethingInRange”.
Change the bool to true if someone begins the overlap and set it to false if he stop overlapping.