Decal shows right in the play mode, but wrong in the launch mode.

I am working on RTS-style game. So when i select a soldier, a circle will appear under the soldier. Just like StarCraft2.
I use decal to do this job. And actually it works in the play mode, but problem is when i play the game in the launch mode, the decal looks wired. How to solve this? Thank you in advance.