How does "Flush" work in XCOM?

I'm having trouble understanding when the Assault ability "Flush" works and when it doesn't. From the XCOM wikia page:

Fire a shot that causes enemies to run out of cover. The shot is easy to hit with, but does reduced damage.

That page goes on to say that Flush adds +30 to Aim, halves the damage, and that even on a miss it can force a target to move. But I've tried it several times, and the Muton at the business end hasn't budged.

Are there certain enemies this doesn't work against? Are they protected from moving if they're in full cover, or they have cover from two directions?

Solution 1:

Flush works as stated on the wiki page and there should be no situations where the cover will prevent a flush effect from succeeding as the intended function of the skill is to force enemies from that cover. A target that ignores the movement factor essentially violates the intended effect of the skill. Most enemies should be susceptible to the flush attack (I have not tried to flush Sectopods yet) including enemies that doesn't utilize cover such as the Berzerker or the Cryssalid. A Muton under the influence of Blood Call should also be flush-able.

If this behavior was observed only once or twice throughout the whole game I would write it off as a bug, but if you keep seeing it often, there may be an additional factor involved I am not aware of which influences the flush attack.

One situation I can think of that may prevent an enemy from moving out of cover is that the enemy is blocked in by the environment or other characters which would result in there being no available path to use. For example a unit in a corner cover where all adjacent map cells in the movement grid are either on fire or are occupied by other units.