I had this problem too where my baby refused to suck from bottle and will gag at it. Persistently offer your baby the bottle nonetheless, from once a day, then to twice a day, so on. A little tip that works on my baby: try to close your baby’s jaw when the bottle’s teat is in his mouth, so he realizes that he actually can begin sucking and that there will be milk coming out! My feeling is that probably baby forgot how to suck from the teat as it’s different from our nipples, so we need to train them! Hope this helps!