Actually, when I first got Amir, he wouldn't even bother with the FDB. He'd on eat pellets. But after a week or straight pellets. I would give him maybe 3 pellets, and FDB. Eventually he learned that they were food and eat one. Ever since then he's loved him.
Could just be possible that where your betta came from, the breeder didn't use FDB or anything of the like, so your betta doesn't even know what it is yet.