I'm not pinning anything on a rookie TE when they don't throw to the one they have. Having a player is one thing. Having that player on the field is two and having the QB actually throw that player the ball rather than run or force it elsewhere is a third.
I'm really curious, listening to Beane talk about Kincaid as a flex TE, what's the actual difference between a flex TE and a slow slot WR? If he's not going to block you don't have to treat him like a TE. He's also 6'3, not 6'6 like Gronkowski was so there are a lot more DBs that can handle him.