Why doesn’t Mina catch up on to what’s happening to Lucy?

In my opinion, Mina cannot even comprehend what is happening to Lucy. Lucy's condition and actions are inexplicable..... none-the-less, Mina doesn't have the slightest inkling that they may be linked to the supernatural. The supernatural is not a reality in Mina's world, and when she finally gets closer to truth, new of Jonathan completely distracts her.