When LO was one month, I started giving her 4oz. of formula once or twice a week (if we were out somewhere as I wasn't 100% bfeeding yet). Then I got mastitis and was doing a 50/50 of formula and bfeeding. I would also pump milk and then feed LO through a bottle with no problem at all. When mastitis was gone, I was able to bfeed again and have been exclusively. Recently, I 've had to leave LO with my in-laws for a few hours with formula AND breastmilk and she wouldn't take the bottle! Downright refused it by blocking her lips with her tongue so the bottle couldn't be put in her mouth! I've tested this out at home - had breastmilk in a bottle and tried to feed her but she wouldn't take it. But she gobbled up my boob.

Anyway...I thought it could be the nipple of the bottle. Any bottles you prefer?