I wouldn't be too worried hun. Ds2 was like that and there was no other explanation for it other than he was just a quieter kid. He didn't start talking until he was about 2.5. He had fantastic comprehension and was fabulous at everything else except for actually verbalising. He would hum the words rather than speak them. Now he is a smart little kid who rarely stops talking but he is still quiet and shy. My girls are complete chatter boxes and I think that is because they all mimic one another. DD2 copied DS2 and DD3 copies the both of them.
If you do have concerns though, the first stop is usually a hearing test. Then a speech pathologist. All the best. I'm sure your DD will be fine and chatting away in no time