Make sure 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 nipple, roll the tip of the nipple between the thumb and forefinger to loosen any build-up. Wash nipples using a mild detergent and water after each use, rolling the tip of the nipple between fingers during washing will ensure to clean out any leftovers.
It can also happen that the air valve(s) at the sides of the bottom of the nipple sticks together. In that case the nipple will collapse or drawn into the bottle. By pulling at that area the valves will open and air can keep flowing through it again.
If the drinking hole appears to be bigger or shows any other sign of damage or weakness, replace it by a new one. For hygiene reasons, we recommend replacing nipples after 3 months.