No consistent words at 15 months

by Jennifer
(New York, NY)

My daughter is 15 months old. While she has at some point said several words appropriately and more than once (including hi, bye, no, yes, and juice), she says absolutely no words consistently, including "mama" and "dada."

It's not as if she once had words and then they disappeared - they just seem to come and go.
One day she'll use several words, and then she'll go several days without using words at all, and then she'll try a new word or two.

Other kids her age say "hi" and "bye" each time and use "mama" and "dada" as names. My daughter only uses "mama" when she is upset or wants a hug (and not every time then); she uses "dada" more often, but it seems more like a babbled syllable associated with her father than an actual name.

She seems to understand quite a bit, but she just doesn't talk.

Should we be concerned?