Ready for the next size teat?

[SummerSevern]

Hi. I've posted on here loads recently and everyone has been really helpful.
DD is 6 weeks tomorrow. A couple of weeks ago I mentioned to HV that she was getting really fussy during feeds. She suggested moving up to size 2 teats. I was dubious but gave it a go- which resulted in DD throwing up everywhere.
We continued with size 1 and she seemed to get better. However, recently she's been getting fussy again - shaking her head away from the teat etc. And she's falling asleep after only 1.5oz, having previously been a really good eater.
So is she ready for the next size, do you think? I'm reluctant to try due to the epic sick last time. Plus, she still sometimes chokes and splutters on the size 1s.
WWYD? And what's the best way of introducing the new size?

Just bumping for you

Try vari-flow teats.
Check there are no lumps in the milk. DS2 would get the rage with these, then decide it was too hard and nod off.
Any chance she's got reflux (often causes fussy feeding)?

Hmm, there are a couple of things you've said that make me wonder about tongue tie. Has she been checked for it by someone who knows what they are doing?

list of symptoms further down

We've had two experiences of this. When DS was 3wo he would take ages to drink and couldn't take in as much as he used to as he would fall asleep before finishing. I was a bit nervous about it seeming too soon but we tried the next teat size up and he was back to normal.

At closer to 3mo the same started, so we tried the next size up again but the milk just flooded his mouth a bit. We continued with the original size but the resistance got worse and worse, so we suspected a reflux issue (he was originally diagnosed at a few weeks old, but just due to vomiting which had been treated with infant Gaviscon and then diminished significantly). We were right and he had to go on to Ranitidine until 7mo.

As an aside he has stayed on the second size (Size 2, Avent Natural) since 3wo and is now 9mo! He has been happy with this ever since the initial change - so just to say it's not always necessary to go up the sizes.