formula fed, 9 month old baby girl refuses bottle. Low weight gain

Birth weight of our little girl was 7lb 5oz. At six month she weighed
13lb 8oz.
She is into 9 th month now. For last couple of weeks she is refusing
to take bottle. She is formula fed. She is barely drinking 12 oz of
formula. Occasionally she spits. We tried mixing flavours like
vanilla, chocolate, strawberry. But no use. She takes 3 ounces of
formula at a time. She takes solids of up to one jar a day. In the 7
the month, she used to drink 28 ounces of formula and eat 1 jar of
solids. So suddenly she has stopped taking formula. Doctor is worried
about dehydration and said that she should at least take 20 ounces of
formula a day and NOT 12 ounces. Otherwise she is active, crawling and
stands with support. Her weight gain also does not seem to be in
correspondence with her birth weight.
I think if we we give her more solid she would definitely take it, but
she might skip on formula. Thats why we are avoiding giving her too
much soild. Though she is very interested in our food, whenever we
eat.
Doctor has put her on Zantac to see if that improves her appetite.
Does any one know, how we can improve her appetitie for formula? Or is
there any medicine we can try?
Note: We use Enfamil Lipil

Thanks a lot
ur