What voltage is the 12V battery after being OFF for several hours ?
Measure in the car on the actual battery posts, with terminals connected,
It should be at least 12.5V.
If its 12V or less it's not being charged.
With the car ON it should read at least 14V, indicating it's being charged by the traction battery.
If not, is it 14V on the actual terminals, indicating poor contact between the terminals and the battery posts ?
Maybe a faulty new battery ?