Are you sure you used a "do every" instead of "do after" code? If you want, upload the game to Forge and send me a PM with its name and I'll take a look at it to see what's wrong.
In the Game Center, go to Collision Groups, and make sure the collision for your enemy group (actors, if following the tutorial) to itself is BLUE not GREEN. That will make sure they don't hit each other :)
Did you make sure to use a "do every" loop instead of "do after"? Also, are you sure the behavior is attached? If you want to, upload the game to Forge and send me a PM with the name, and I'll try to see what's going on with it.
 "Stencyl Tutorial 4: Mouse-Based Shooting"){kind=icon}
 "Stencyl Tutorial 3: Attributes & More"){kind=icon}
 "Stencyl Tutorial 2: Attributes, Etc."){kind=icon}
 "Sonny"){kind=icon}
 "GemCraft"){kind=icon}
 "Quiz of the Living Dead: Zombie Analyzer"){kind=icon}
 "Stick War"){kind=icon}
 "Elona Shooter"){kind=icon}
 "John Citizen"){kind=icon}
 "Lock-n-Roll"){kind=icon}
 "Pinch Hitter 2"){kind=icon}
 "Doublue Wires"){kind=icon}
 "CycloManiacs"){kind=icon}
Are you sure you used a "do every" instead of "do after" code? If you want, upload the game to Forge and send me a PM with its name and I'll take a look at it to see what's wrong.