Mirroring not working correctly - need help

I followed the instructions last time on how to mirror my tube but when I tried it this time for my tube support rings, it didn’t work. It basically moved the duplicate to the other side but didn’t actually mirror it.

Here are the pics: http://imgur.com/a/PcKZR

Thank you all for helping me out!

The object has some rotation as you can see in the properties panel. Apply the object rotation before mirroring (Ctrl+A)

Thank you!

For future reference, also apply any scaling and make sure your object origin is in the middle.