Nipple size up

Hi folks,

I have two preemie twins that are currently two months old born at 34 weeks. They spent 5 weeks on dr brown preemie bottles then moved up to size 1 dr brown when they outpaced the preemie. One week after we moved them into evenflow balance slow flow bottles. They adjusted and took it very well. 3 weeks into the evenflow wide neck slow flow nipples I’ve started to notice the nipple collapsing and extremely strong sucking from them. I tried the pinch/checking the valve but it still seems to happen. Is it time to move into the medium flow nipple? Does 2-3 weeks seem little crazy to move from slow to medium?

Also what options do I have on medium flow nipples that work on evenflow wide neck bottles? If the bottles were hard enough to get the medium flow nipples are even harder.

Thanks