Its likely because of her birthday. You will need to review her details and make sure you selected the child lived with you all year. Changing this should then make her eligible.
There is a special rule for children that are born during the year. If the child lived with you for more than half the time they were alive, then you can select that "this child lived with you all year" in the dependent information section. If you didn't select this option and your child was born more than 6 months into the tax year, then you won't get credit for the dependent on your return. So, return to your Personal Info tab, and click Edit next to your dependent and verify this information.
