Hey-hope you dont mind me gatecrashing as I am a FF mummy-ex BF!
We have the Breast Flow bottles for Brooke, she has only been on the stage 2 teats for about 2 months. They flow sooooo fast!
I was worried about the clear teat collapsing at first too but then I realised it was actually meant to do it as its mimicking what your breast does. One thing I would say is keep checking the grey inner teat as Brooke's have gotten holes in so many times and Iv had to replace them.
I guess all you can do is keep trying your LO with the new teats to see when she can cope