If the 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.
If the hole appears to be bigger or shows any signs of damage or weakness, replace it with a new one. For hygiene reasons, we recommend replacing nipples after 3 months.