she is only two and they can take a while to get all the letters I wouldn't be hugely worried unless it was almost all of her words. Usually there are a few letters they get stuck on by the time she is in kindergarten it should correct it self

my nephew marcus had a bit of trouble with his speech, my sister found that gettin him to interact and talk with his older sister jayla and also other kids his age encouraged him to talk more and therefore his speech improved...my sister did not want him to start going to play group until he was 2 and a half 3 but she decided to start taking him 1 day a week at the age of 2 and having him constantly surrounded by children his age encouraged him to talk as he would want to communicate with them and he cant do this if he cant talk...all the best with your daughter but like my sister you should follow your instinct and get it looked into