Have you followed the instructions in the manual for troubleshooting?
Which camera do you have? Are you using Liveview on your camera? Many 3rd party flash or triggers will not work with Canon Liveview.
Higher end cameras like the 60D, 70D, 5D, 7D etc all have an option in the camera menu to disable "Silent Shooting" while in Liveview. This allows the trigger or flash to work while in Liveview.
The 550D and other "Rebel" series cameras do not have this option and there is no easy way to make your flash or trigger work while using Liveview. You must then use the optical viewfinder.