I had similar problems ahead of me going back to work. Things to try are different teats and bottles. I found the Tommee Tippee closer to nature ones good, though I think every baby is different. Another thing to try is using a cup.
Varying tips I found on different websites included try to feed them the bottle when they are not too hungry (so give them the bottle earlier than you would normally feed them), offer them a bottle when they are sleepy as they are more relaxed. I will post more as I think of them. Perseverance is the key. To start with Ellie would only take the bare minimum from a bottle 1-2oz. Now she is down to 1 bottle while at Nursery and twice this week she has had 7oz from the bottle!