I agree that you need to take her tp the doctor. Sucking skills may be affected which is not good since its important for infanrts...maybe it could be the nipple of the bottle...
Have you tried to change to a few different types of nipples on the bottle? She needs to be on baby formula not regular milk at this age because regular milk does not have the nutrients an infant needs to grow. If she still not able to suck with the different nipples, then have your MD get her evaluated for sucking skills. Infant sucking mechanisms do not start until about 30 weeks in the womb and sometimes later.