I was wondering this same thing. My DD is 7 1/2 months, and cannot sit unsupported. I sit her in between my legs and she will rest on me, but when I let go, she starts tipping to the side. I have tried teaching her to put her hands out in front of her, but when she does this she just gets interested in picking up a toy and then sitting up and leaning back on me. I really don't think anything's wrong, I just think she's a bit lazy about it. And I'm not willing to take the chance of just putting a pillow behind her, as she tips from side to side still.