Make sure that the drinking hole and air valve are not blocked.
If the drinking hole appears to be smaller and the baby experiences difficulty drinking, the top is probably clogged. To unclog the teat / nipple, roll the tip of the teat / nipple between the thumb and forefinger to loosen any build-up. Wash teats using a mild detergent and water after each use, rolling the tip of the teat between fingers during washing will ensure that any leftovers are cleaned out.
It is also possible for the air valve(s) at the sides of the bottom of the nipple to stick together. In that case the teat will collapse or be drawn into the bottle. By pulling at that area the valves will open and air can begin flowing through it again.
If the drinking hole appears to be bigger or shows any other sign of damage or weakness, replace it with a new one. For hygiene reasons, we recommend replacing teats after 3 months.