Is my daughter falling behind?

I think it is reassuring that she understands and can follow out commands the fact that she is an only child probably contributes as she is not around a sibling to talk to. Try taking her to preschool or playgroup to get her to interact with other children I think that helps heaps in children's development. Also and I know many parents won't agree but kids tv is great too I have found my daughter has started saying so many more words from watching programs like Dora or Diego she isn't a telly addict but when in the house I have kids tv on in the background and I find that has helped a lot. Also reading to them and getting them to get involved by asking what's that and see if they can say stuff like puppy or rabbit stuff like that. I think you are doing the right things as she does understand you'll find one day she'll just start talking away but if you are worried try some of the things suggested good luck but i think you have nothing to worry about really