We used avent bottles and teats and never changed on their schedule and were usually a couple of months behind the suggested age when we finally did change.

I was told that when a baby is sucking too hard on the bottle their puking will get worse... so we used that as a sign that it was time to change. The one and only time we changed according to the avent "schedule" we ended up with a very unhappy baby who wasn't able to swallow his milk fast enough.

I'd go back to the two hole teats and see if that helps.

BW