My DD2 (18m) has a very limited vocabulary. Can say about 6 words and that's it. She's very laid back and seems like she never really tries to pick things up 
It doesn't help that DD1 (3) was very advanced - walked at 8 months, talking from a year old and just picked everything up so quickly.
He motor skills seem fine - walked at 12 months - and she has good eye contact and is very cuddly but she just doesn't seem interested in talking.
When do you start to worry that there may be a problem?