Light flash attribute doesn't work when light is already on

I’m trying to use the “flash” attribute in a script.
I tested the command to flash the lights but it seems to only work if the light is off, if it’s already on, it doesn’t flash…

Any extra line in the command I could use to force flash even if it’s on already?

Update:
***Sorry never mind, It was my mistake, seems flash doesn’t work on all lights…