15 books? You're still well on your way. I'd say about 50 or so until it becomes second nature (photoreading all and activating many, not necessarily all).
Photoreading it again and again won't necessarily help. Try to give yourself some more time before activating, that is, more incubation. Ask mind probing questions and search for an answer before activating (in your head) and then begin looking for answers in the book.
Also make multiple passes over the book when activating. That really helps.
