Model Blueprint for Hounds not dropping?

[(PSN)The_Shade_Caster]

I'm just wondering what is the drop rate for the Model Blueprint for Hounds from the Sisters? I have hunted and killed 8 Sisters so far and at minimum get 3 blueprint(unidentified item) drops when I do a public mission to kill them. So that's 24 blueprints I have obtained and not one of them have been any Model Blueprints(head for hound). I'm stuck being unable to make my own custom one because of this and just want to know if this is a glitch or if the blueprint just has an abysmal drop rate. 

[Dauggie]

they do drop, i never had to build it since they come prebuilt after sister kill, but i do noticed some head bp or it could be something similar. haven't checked legs, but see if he has those bps

[DarknessNightshade]

The official drop tables don't seem to mention specific drop chances for the Sisters or their Hounds, and seems to be missing from their database thread here: 

If I was gonna assume how it works, each of the 12 different parts you can get via Killing a sister have an equal drop chance (around 8.3%), based on you only having acquired 24 parts, I can only assume you've not done enough runs to get one of each part.

[(PSN)AbBaNdOn_]

Im about 6-7 liches in and have never seen a head.  

[(PSN)crashteddy03]

2 hours ago, (PSN)The_Shade_Caster said:

I'm just wondering what is the drop rate for the Model Blueprint for Hounds from the Sisters? I have hunted and killed 8 Sisters so far and at minimum get 3 blueprint(unidentified item) drops when I do a public mission to kill them. So that's 24 blueprints I have obtained and not one of them have been any Model Blueprints(head for hound). I'm stuck being unable to make my own custom one because of this and just want to know if this is a glitch or if the blueprint just has an abysmal drop rate. 

Just some bad RNG luck I'm afraid.

I've taken down 5 sisters so far and have seen all of the model blueprints, but only 2 each of the cores, legs and tails. Oh and the first three sisters had identical hounds - as in all four parts identical!