Sometimes the anti-colic valves on the TT CTN bottles get stuck shut. After you wash the teat, gently squeeze the edges of the valve (from the sides) so that it pops open. That can help stop the teats collapsing.
The TT size 1 teats are so slow that it's unreal. Our little one was on the stage 2 teats at 2 weeks old, and has been on them ever since.
If you're not using TT bottles, then I'm not sure! Although my daughter did stay on the size 2 teats no matter what bottle it was, after we'd made the switch from TT.