Containers dont die

Hi All,

I’m using CoreOS cluster and manage the units with fleet.
I started to get some kind of unwanted behavior of some containers.

I load a unit and when I destroy that unit, the container stays alive.
I have the following code in the unit:

ExecStop=/usr/bin/docker stop CONTAINER_NAME

I want to stop it so it will finish what is currently active and then die.
But it never dies!
Checked the journalctl and got this:

systemd[1]: pushserver.service: Control process exited, code=exited status=4
systemd[1]: pushserver.service: Unit entered failed state.
systemd[1]: pushserver.service: Failed with result ‘exit-code’.

How can I check where is the origin of this issue?
docker version is: Docker version 1.8.3, build cedd534-dirty
AWS CoreOS version is: CoreOS-stable-766.5.0-hvm (ami-55d20b26)