My DD#1 was stringing sentences together by 18 months. My 15 month old is saying single words at moment...so we will see how she goes by 18 months....BUT i do have two nephews and by 2 years old they werent saying much either....single syllables...would point to things, grunt,etc....not actually putting sentences together....by about 2 1/2 years of age it all suddenly clicked for them....they started saying complete sentences....e.g "drink mummy please" or "where daddy?"
As others have said, maybe get her ears checked out etc.....but give it another 3 months or so...you never know, it may just fall into place for her too.....