I have the same problem also but i changed to glass bottle and the problem solved. As for the flow, i usually cut the hole bigger by my own.
But the newly ones are hard .... baby like to suck the soft one.
I let my b tried pigeon and avent..he does not like it and cry and i have no choice to stick with Nuk. But the teat 越 "sup" 越硬. Lat time the teat 越 "sup" 越soft. dont know what happen to the stock now..越 "sup" 越硬.