My daughter is starting to speak at 18 months and she has picked up many words recently but she is refusing to acknowledge me. She can say YeYe (Grandfather), Ah Ma (Grandmother), apple, orange, yellow, baba (Daddy), Elmo and can repeat many words after us but she is just refusing to say 'Mama' (Mother) which I'm asking her to call me that. I am so upset and affected because I spent the most time with her. Is she doing this on purpose and does this happen to any other mums?

Lol! No matter how tiny kids are, they have the talent to drive all sorts of emotions out of you. I doubt she would be doing it on purpose, but yes, she may not be liking to follow the instructions. She is outrightly telling you, do not tell me to do this. Or may be she is already addressing you with something that she has formed of herself. As you mentioned he calls, grandfather, YeYe. One cannot make out what YeYe is. Same way, she may be calling you something and since you only expect mom to be called, maama, maa, mommy or any word starting with M. Perhaps, you are missing your name that she is calling you for long now. ;p