I noticed not all avaiable ingridients are shown when choosing the desired effect.
Like when I choosed: weeknes to poision, weakness to schock and ravage health nothing came up but it is possible to brew such a potion.
Void salt + Giant Lichen and Skever tail -> 97% weaknes to shock 30 sec, 67% or 65% weakness to poison 30 sec, 97 dmg )
I tested it in game after getting the right ingredienc effects from http://skyrim.melian.cc/?cmd=cmdSkyrimIngredientList
Can you please recheck?
From the ingredient list i have the following effects for the 3 ingredients you posted and check which effects they actually have in game?
Void Salts: Weakness to Shock, Resist Magic, Damage Health, Fortify Magicka
Giant Lichen: Ravage Health, Weakness to Shock, Weakness to Poison, Restore Magicka
Skeever Tail: Damage Stamina Regen, Ravage Health, Damage Health, Fortify Light Armor
With those ingredients you can get: Weakness to Shock, Ravage Health, Damage Health
but no Weakness to Poison.
So i guess either some of my ingredients have the wrong effect in my database and differ from actual in game values, or you have had too much Skooma mate!
Edit: You might mean these ingredients per chance?
Potion: Damage Health, Weakness to Poison, Weakness to Shock
Deathbell, Giant Lichen, Void Salts