Baby is busy right now. She's learning to sit up, crawl, and walk — or some combination thereof. Perhaps he has a lot on his mind, and his attention may be focused elsewhere. If your baby isn't signing back, consider the following reasons.
Maybe you're overwhelming baby with too many signs
Processing lots of signs at once is more than baby's brain can handle. You should never be working on more than a handful of new signs at any given time. What's a new sign? Any sign that she hasn't signed back yet. So if you're just beginning, work on only a few initial signs.
If you've been at it a little while, and baby has signed a few signs back to you, work on those signs plus a handful of new ones. This method is how, over time, baby builds her sign vocabulary — not by being bombarded with a sign for everything in her world.
Maybe the signs you're using aren't for the things that excite and interest baby
The signs you're introducing to your baby must also be ones he's interested in. He may not be crazy about stars right now, for example. In fact, depending on his age, his vision may not even be developed enough to differentiate stars from the rest of the night sky. So why should he sign STAR?
Many babies are fascinated by the light when they looked up toward the ceiling. Perhaps the sign for LIGHT is a better fit for their interests.
Pay attention to your little one, and he'll reveal his current passions to you.
Maybe baby is trying to sign, but you just don't realize it
Know up front that your baby's initial attempts won't mirror your signs. Consider, for example, the sign for HELP. It resembles a thumbs-up gesture with one hand as it rests in the palm of the other hand. But on a child, it could be an elbow that rests in the palm of one hand while the other hand waves. That gesture happens to look an awful lot like the sign for TREE. So how do you know if the child is signing HELP and not TREE? Context is crucial, as well as the frequency with which the baby repeats his rendition of HELP.
So, pay extra attention. Does she make consistent movements with her hands that even remotely (and it may be very remotely) resemble the sign you're showing her? If so, then she's signing! And she's probably wondering why you don't understand her. After all, in her mind, her sign is identical to yours.