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?

I suspect your little one is messing with you. You can try this (I'm not sure if it will work, but perhaps no harm trying?): When you are not around, ask your husband to point to the family photos and get your daughter to identify who is in the photo (e.g., Where is daddy? where is yeye? who is this *points to ah ma*? who is this *points to you*?/where is mama?) Just an idea.. Don't fret over this. Once she starts calling you, there may be a day where you wish she is calling for everyone else but you.. ;)