B.,
Please don't use a nipple shield. It can affect the way your baby nurses and can cause your milk supply to drop. I would recommend, like the most recent post, that it not be you giving the baby the bottle. I would also stop offering her different nipples - pick one and stay with it. Babies have natural tongue thrust and will push out when something gets placed in their mouth. Remember that when your breast enters her mouth, your nipple actually molds to the top of her pallet; when you place an artificial nipple in there, no matter how soft, it won't mold the same way, so it's difficult to "get it to work". You might want to look into this bottle called "Adiri". You'll understand when you google it - it's much softer than those other silicone nipples. However, if you want to continue buying the over-the-counter, easy to find nipples, look for a nipple with a large "areola" and a short "nipple" - the avent do not meet this criteria - the playtex, a bit better. The Avent bottles will also fit a nipple called "Munchkin" - you can find those in more stores, but they have more of the criteria that we Lactation Educator's recommend for nursing babies.
Good Luck.
J.