Delays in speech

I start worrying Mei Mei, almost reach 14 mths, still don't know how to say a single words. In order to heard she speak, we even bribe her with ice cream or bread. When we ask her to call Papa, she'll say Mama, when we ask her to say Mama, she'll say Papa. But I think she know the meaning of Papa and Mama, because when I ask her where is Papa, she'll look at him. She understand our conversation quite well, but the problem is she don't want to talk. Compare to Jie Jie, Jie Jie able to speak 3-4 words sentence at this age. Jie Jie is really fast in her speech. Every child is difference! Hope Mei Mei can pick it up fast and greet us when see us come back from home, not with bribe or force.