I have been using two nipples interchangeably during the same period, however, for one of the nipples, milk does not flow easily. Why is that so?

The nipple hole and the ventilation valve (vent) on the nipple flange could be clogged with milk residues.

When the hole is clogged, milk does not flow easily. This may lead to situations whereby the nipple collapses and caves in towards the bottle.

To prevent nipple collapsing, keep the nipple hole and vent clear so that they will not be clogged with milk residues.

When you wash the nipple with washing detergent, gently use your fingers to rub clean the ventilation valves and vent.

Please be careful not to pull and/or wash the ventilation valve too hard as the ventilation valve or vent may split and cause leaks.