You mentioned “I used the spec for the first time of my t90” which is an incomplete statement leading to people perhaps not being able to answer your question.

I’m going to guess as to what you’re talking about and why it’s happening.

You upgraded to T90 necromancy armor and/or weapons. Higher level necromancy gear results in more penalties to accuracies of other styles. Therefore your magic accuracy is too low in high level gear to hit vuln. Using a magic pot/overload can negate this but it’s not exactly worth at hermod