My first game I couldn't read the books and didn't have many problems with the summoning tests (other than overlooking one conversation needed to set a trigger).
My most recent game I could read them all and had major problems - the mushroom wasn't there at all (I used one picked up from a different act) and the summoning 1 master didn't accept my ingredients and I had to drop them and pick them up again.
Linguistic skill doesn't seem to be the culprit.
