Doom
 

The target is supposed to receive critical attacks every time, but this does not appear to be so. What gives?

Some things do not count as attacks but instead as skills, i.e. the elven bowmen double shot. These do not seem to work, likewise with emerald dragons special attack. So these skills is what it does not work on, I dunno maybe it's suppose to function in this manner and maybe it's not.

Cast it on enemies elder vampires to see true Doom.

Hehe, pity you can't use doom on your own ancient vampires.

My recollection is that the spell description specifically mentions that this affects only BASIC attacks. So, I'm pretty sure it's working as intended, if the OP is observing the lack of criticals per a skill.