I was in exactly this position last year. Started out exclusive, she pulled back, we were FWBs for a while, but she continued to talk as though she wanted something more. Turned out that she did want something more, but she didn't want exclusivity. She wanted affection and quality time and emotional intimacy and to make travel plans and pursue the goals we had in common -- and she also wanted those things with other men at the same time. She is so attractive that she can pull in anyone she wants to, and she simply didn't see a reason to cut off any of her various suitors. Each of us offered her something the others didn't, and sexual variety is important to her. Honestly I can't really blame her -- if I could be as successful on the dating market as she has been, I'm not sure I wouldn't make the same decision.

I don't know what's going on with your FWB, but in my experience it's entirely possible for someone to want to be more than friends but less than committed partners.