IIRC, opposing defenses seemed to have had Fitz figured it out and were crowding the Bills receivers at the line making it difficult for him to complete those quick slants. Since he didn't have the arm to throw deep, he was becoming less effective. I was surprised with how effective he became with Houston, even hitting D.Hopkins on big plays.
Now with the Jets, Brandon Marshall and Decker are both averaging over 13 yards per reception so Fitz appears to not be limited. Don't know how many of their yards are YAC though. They are also bigger more physical receivers that the Bills didn't have when he played for them.