If you really had a 20% drop rate, consider yourself lucky.
I had to kill around 250 Adonian Spirit Hunters to get a complete pattern. Out of 10 drops I got 6 C-patterns, 2 B-patterns, 1 each of A & D.
Plus, I found all of them on the platform in E Adonis, N of the Redeemed Temple. Hunting the Spirit Hunters in the SW (S of the Incarnator) yielded no pattern at all after ~60 kills.
On a side note, I completed both an Adonian Soul Dredge pattern and a Viscious Visitant pattern before I got the last part for the Infernal Demon blueprint. So if you're out for these, you know where to look