When should I up the teat size?

[dennant]

Hi, baby is 12 weeks and mix fed using tommee tippee closer to nature bottles. Still on size 1 tests and wondering when I should up to the next size. Any idea what signs to look for?
Thanks in advance

If mixed fed they normally say stay on slow flow.

If they are struggling to get enough out or the teat collapses.

Oh yes, unless it is causing real issues. Should have added that.