Some of the reasons cold be associated with:
Not enough time for battery to be activated/charged after removal of the tab.
Dented battery surface causes poor contact with battery terminal of the hearing aid.
Battery is dead (even if it is brand new). This is very rare. Can as well happen when not activated correctly. Built up dirt on the battery terminal of the hearing aid causes poor contact.
The battery does not fit into the battery door of the hearing aid (some battery doors are designed with a smaller cavity where the battery negative housing is placed).
If the hearing aid does not work properly after replacing with a new battery (wait two minute after removing the tab before use), please take your hearing aid to your hearing care provider for further troubleshooting.