Sigma EF-500 would not flash twice sometimes.

[Sleepy]

Hi,

I have a issue when using my Sigma EF-500DG Super.
As I know, in P-TTL, the flash would flash twice. The first flash is for the camera to measure the proper power output based on the reflected light the camera receives, and the 2nd flash is the one which fills the light.

However, I found that my Sigma EF-500 DG Super would not fire the 2nd flash.
It happens even if the batteries are fully charged.

Does anyone have the same issue?Is it normal?

Thank you very much.