Trigger enter and directly Exit

Hello,

I develop a Tower Defense and I have a problem with the Trigger... I put a trigger colider to my tower and Monsters. The problem is : when the Tower detect an enter trigger he shot the enemy and then the monster exit, the tower stop. But when I have a little more monster the tower detect the enter and the exit simultaneously !

I put the monster in my range in a tab to always shot the first enemy and I put a message when the monster enter and when the monster exit the range (with monster's number).

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Thanks all. I hope you can help me :)