I have the same issue with the weapon emote, but i dont play with mods.
Sometimes i can do them but most times not.
Also so far i had next to none fatalities, when the NPC enters the stunstage i am unable to click left or right mouse button…my character is just standing there and watching…
I have not discovered if there is the connection to the weapon emotes, i just stopped using them since they dont work at least half the time.
One of the reasons i never buy emotes at the basar even though some are quite nice…