You have to go by the baby and not what the packaging says, my two needed faster teats fairly early (I used vari-flow teats for DD as the slow was too slow but the medium a bit fats), some babies stay on slow flow until they come off bottles.
If she has a strong suck then the slow flow is probably too slow for her, mine used to get noisier when the teat was too slow and they used to make the teat collapse. Getting tired after only a couple of oz is also a sign she is having to work too hard to get the milk out.
Laura
