I use Aptimal hungry baby and sze three Advent teats. When I wash them before sterilising, I fill them with hot water and push my little finger into the inside of the teat to check the water jets out the three holes. Not sure why yours would be blocking other than maybe the milk had a few little un-mixed lumps in it. I mix mine after the 30 minutes (when kettles boiled) then let them cool.
My only other thought is faulty teats? But if its happened on size 2 and 3 that would seem an odd coincidence.
Sorry I can't be more help xxx