Smoke detectors lose sensitivity over time. The longer a device is in use, the less likely it will be picking up very small amounts of smoke. This is due to how the sensing element functions.
It is not unusual to have a new smoke detector be more sensitive than the replaced one, especially if the old one was approaching or past its expiration, so that is likely what is going on here. If it is very close to or in the kitchen, generally it is recommended to adjust the location.