I was not able to bf so I´ve been using the bottles since day 1 and there has not been any issue with them. She use them even with cereal at night. She uses level 3 nipples since 2 months as her milk is for refluxers. Are you using the tiny brush for the small tubes, maybe smth got stuck there. Try using #3 nipples, or are you sure the nipples are the correct number for the age, as they are similar you can get confused.