Toddler talking

Hi mommy, I wanted to ask your opinion if my baby is on track on her language development. She is 17mos old and can talk 7-8 words already with purpose and can understand and do most of what we asks her to do. However my concern is she won't call me "mama". I keep on teaching her but get nothing into it. Should I need to be worry about it? Thanks in advance for your thoughts.